Why does ... Japan get a pass for supporting the Axis in WW2

Japan doesn't get a pass. The Japanese government is routinely criticised for failure to apologise to and/or compensate those who suffered due to Japan's actions in WWII (and the Second Sino-Japanese War) and Japan's colonial policies. This is despite multiple apologies by people in the Japanese government - many believe that the apologies are insincere, too unofficial, or simply insufficient.

Such criticism of Japan is most prominent in countries occupied by Japan during the war, especially China (PRC) and South Korea. However, it also occurs outside the region. Some of this is related to the Japanese occupation of much of East Asia and SE Asia, such as the US House Resolution 121, stating that

the Government of Japan

1. should formally acknowledge, apologize, and accept historical responsibility in a clear and unequivocal manner for its Imperial Armed Forces' coercion of young women into sexual slavery, known to the world as `comfort women' , during its colonial and wartime occupation of Asia and the Pacific Islands from the 1930s through the duration of World War II;

4. should educate current and future generations about this horrible crime while following the recommendations of the international community with respect to the `comfort women'.

and some is related to mis-treatment and murder of Allied POWs and interned civilians.

Japan is also routinely criticised for inadequate/inappropriate teaching about Japan's role and Japanese atrocities in the war. This criticism comes from both inside Japan and outside Japan. The Japanese government's approval of the textbook 新しい歴史教科書 (New History Textbook), which ignores or whitewashes many Japanese atrocities and war crimes, in 2001 was publicly criticised in a series of anti-Japanese protests in East Asia and SE Asia in 2005. As an example of the content of the book, the 2005 edition's discussion of Japanese repression and atrocities in Indonesia, with about 300,000 deaths, is limited to

But when war neared its end and food was scarce, Japanese military police sometimes forced locals to do harsh labor, and were cruel to the local people in other ways as well. The fact that the Indonesian language contains not only Japanese loanwords like seishin (spirit), but also romusha (laborer) and kenpei (military police) reflects the complex situation of the times.

Japanese government instructions to tone down the Japanese Army's role in the widespread "suicide" of Okinawan civilians during the Battle of Okinawa resulted in large protests in Okinawa, and protests by the Okinawan government, in 2007: http://news.bbc.co.uk/2/hi/asia-pacific/7020335.stm

Concerned individuals in Japan, including teachers, also protest about these things.

And I'm 99% sure Japan killed more civilians in east Asia than both atomic bombs.

Far, far more. About 4,000,000 in China and 2,000,000 in SE Asia, not counting disease and famine deaths. Famine deaths would add about another 10 million. Note that it's often difficult to assign all blame for a famine to one combatant, but the north Vietnamese famine near the end of WWII, which killed 1 to 2 million people, resulted quite directly from Japanese action.

References:

US House Resolution 121: https://en.wikisource.org/wiki/United_States_House_of_Representatives_House_Resolution_121

English translation of 新しい歴史教科書 (New History Textbook): https://web.archive.org/web/20050906080746/http://www.tsukurukai.com/05_rekisi_text/rekishi_English/English.pdf

For more detail on deaths in occupied Asia, see https://www.reddit.com/r/AskHistorians/comments/k4hc1s/did_the_dropping_of_the_atomic_bombs_on_japan/

Without any need to indulge in disputes over the merits of Italian historiography of "Nazi-Fascism", it's probably worth pointing out that Italy did surrender when it became clear that the war had been lost - at least in so far as any sensible objective it might have wished to pursue at the beginning of the war.

While this - not especially agile - attempt to leave the Axis and to reclaim a somewhat independent "democratic" agency is often portrayed as merely opportunistic (both by Fascist apologists and by the fiercest critics of the establishment), it did nonetheless produce a series of positive results. It limited the number of casualties for both the engaging armies, Italian and Allied, even if a large portion of the Italian enlisted men was eventually moved to Germany as forced labor. It provided evidence that (not irrelevant) portions of the Italian establishment were willing to cooperate not only with the "victors", but with those forces which - in that specific context, and at times with severe limitations - represented ideas of practical and aspirational democracy, leading to a real improvement in the lives of the Italian population, as well as to better (albeit not perfect) forms of cooperation with the Italian neighbors. It distanced the Italian institutions - it very modestly distanced the Italian institutions (and such a distance can't be really invoked, except for a few groups and individuals, for the crimes committed during the Italian colonial occupations) - from the universal perception of the worst horrors of WW2, proving that the Italian "people" - as loose as this conceit is - wasn't always, and entirely, supportive of the "fascist project" for a new Europe.

In this sense, Italy does get a pass - even if, without insisting in semantics, this is certainly not the way one would address the issue in a proper historiographical context (and, incidentally, it's a rather flippant way to discuss the events occurring during and around WW2).

At the same time, Fascist Italy was - intentionally - one of the two pins of the Axis. The term itself, the "axis" running through the core of Europe's balance, had been coined by Mussolini and, while it represented perhaps a rhetorical artifice more than a practical reality, it does speak to a Fascist intent of being an integral part of the Axis itself. There is no "lumping together" except for what the Fascist leadership intended and explicitly pursued. The degree to which the Italian population followed the Regime or was coherced by it is a matter of historiographical debate, but doesn't change the substantial fact of Italy's role and participation in WW2. Nor the fact that the new Italian leadership, after September 1943 and especially after the democratic transition in 1946-48, was able to re-position Italy in the international context and among allies which did not - at the very least - pursue the intentional murder of millions of men, women and children.

As to other Axis partners - which is not my specialty - it is my understanding that all of them have their own controversial and complex legacies, their episodes of "historiographical amnesia", and their unresolved international disputes. It's hardly surprising that Italian historiography of WW2 deals more with Italian Fascism than with the Japanese occupation of China and East Asia. Even in that context, I do not remember the Japanese crimes being overlooked, or subject to particularly favorable revisions.

As to the Soviet Union - it's frankly impossible to draw any parallel between the alliance between Italy and Germany and the brief opportunistic interlude of the Ribbentrop-Molotov pact - it's various actions before, during and after the war have been thoroughly documented and, to my knowledge, subject to a long and deep historiographical enquiry. And legitimately so.

Neither - nor this sorts of appeal to the "faults of the other" - help us get a better sense of what Italian Fascism did, what it aspired to do, why it did; of what role it played within the context of "generic fascism", of its relations with Nazi Germany, of its "epochal" and possibly "trans-epochal" significance.

Last, I would really encourage any reader to keep in mind that almost every form of historiographical inquiry is bound to bring you a bit closer to a dimension of the past which is inhabited - for the most part - by the dead. History is not always a graveyard, or a sanctuary; but, sometimes, I think one ought to approach it that way. Certainly, the dead did not live and die to solve our disputes, or to weight against one another on imaginary scales.

As general reading, I'd recommend (there's many others - let me know if you have some specific topic in mind):

Kallis, A. - Genocide and Fascism: The Eliminationist Drive in Fascist Europe and Fascist Ideology: Territory and Expansionism in Italy and Germany 1922−1945

Pavone, C. - Una guerra civile. Saggio storico sulla moralità nella Resistenza

Of interest, even if I have yet to read it through, might be Bosworth's Explaining Auschwitz and Hiroshima - which probably addresses parts of your question from a more general angle.