From an earlier answer of mine

Some things need to be clarified here. One, the national flag of Japan, the hinomaru is not the same one as in the OP's panty-link. The rising sun flag only has a limited relationship with the postwar Japanese state, mainly its use by the Self-Defense Forces. This has not been without controversy as we will see below. Second, the Nazi swastika ensign was a political symbol of the NSDAP. Between 1933 and 1935, Germany used a dual flag of both the swastika and the old imperial flag. This was mainly done to please the NSDAP's conservative allies and ended with von Hidenburg's death. Desecration of the the swastika ensign abroad added a further impetus as it was merely symbol of a political party rather than an official national one. The symbology of the swastika was indelibly associated with Hitler and his ideology.

Much of the evolution of of these symbols can be traced back to the occupation period. There are two things going on here that make a direct comparison between the military symbols employed by the FRG and postwar Japan a bit difficult. Firstly, German politics and history gave postwar German politicians a set of alternative political symbols to simultaneously reclaim the past and mark a break from the Third Reich. The black-red-gold tricolor was a symbol of German liberalism and democracy that was very well suited to this task (although the process of adapting the tricolor was not exactly straightforward- this answer of mine goes into some of the details). The German tricolor and eagle were natural choices that could garner a degree of support across the German political spectrum. Japan, in contrast, lacked such alternative symbols. The rising sun and the rayed flag dated back to the Meiji Restoration and its immediate aftermath. There was no analogue to the Paulskirche or 1848 from which the Japanese could draw upon. Moreover, the fact that Japan was not divided, but under a unitary occupation also made the national symbol question less urgent. In the FRG, the adaption of national symbols was deeply connected to laying claim over national sovereignty over the emerging GDR. National symbolism was much more freighted with ideological one-upmanship and competition in the German case. The largely conservative Japanese collaborators with SCAP had no real ideological rivals that could needle them into making a break with the past. The Japanese Emperor's position in the postwar constitutional order did not make it necessary to expunge symbols related to the imperial house, or the various historical and mythological associations with it.

Secondly, the timing of the official establishment of both the Bundeswehr and the Self-Defense Forces (the early 1950s) was a period that was very amenable to acceptance of older military symbols used during the war. Both institutions drew a lot of their major personnel from veterans of the war, and there was a deliberate process of foisting the crimes of the prior regime on various hotheads and fanatics. Thus symbols like the rising sun or the iron cross became disassociated from their unsavory recent past. While open Nazi symbols were forbidden in the Bundeswehr, other elements of the German military tradition survived. Rommel, for example, became one of the exemplars of a German soldier for the early Bundeswehr, with the 21st Panzer Brigade being based in Generalfeldmarschall-Rommel-Kaserne. In Japan, the JMSDF presented itself as the inheritor of the IJN's position as a force for progressive change and modernization. The JMSDF packaged the rayed flag, as well as the preservation of the battleship Mikasa and various celebrations of Admiral Togo as honoring this tradition of progress. The conservative political turn in both the FRG and Japan in this period favored such a selective reading of the nations' past and their symbolism. Popular culture in this period tended to portray regular soldiers as heroic victims of militarism or Nazism. War films produced in Japan and the FRG of the 1950s and 60s tended to portray soldiers of both regimes as victims not perpetrators. While there were exceptions to this trend such as Kobayashi's The Human Condition trilogy or Wicki's Die Brücke, most of the cinematic portrayals of the war hewed to eulogizing the futile heroism and pointless sacrifice of the Axis soldiers.

It bears stressing that postwar attempts to decouple these symbols from the wartime governments was not entirely successful. Memories of the Pacific War ran long in East Asia and the SDF's use of the rising sun emblem has created controversy abroad. As part of the wider post-Cold War turn, the use of SDF forces in UN-sponsored peacekeeping missions summoned up ghosts of the war. The SDF's deployment to Cambodia in 1990 brought about the public concerns of a number of East Asian intellectuals and leaders. China's President Yang Shangkun expressed his concerns about the mission, using the ensign as a focal point:

the most extreme foreign critics suggest that a SDF overseas peacekeeping dispatch could be the precursor to Japanese forces marching abroad under the battle flag of the Rising Sun.

When the civilian UN volunteer Atsuhito Nakata was murdered in Cambodia, his remains were returned to Japan wrapped in a UN flag, not a Japanese one. The use of the rayed flag by various Japanese ultranationalist groups also creates an unsavory association for the SDF and is still an issue as Japanese politicians to grapple with as the SDF moves outside its native territories to places like Iraq and Afghanistan (but we're crossing the 20-year rule). In united Germany, the Berlin Republic has taken a much firmer stance than prior governments against honoring traditions and symbols that were used by the Third Reich. In 1998 the Bundestag voted on the anniversary of the Guernica bombing that veterans of the Condor Legion should not be honored in the Bundeswehr, leading to a protracted renaming process for Jagdgeschwader 74 whose namesake Werner Mölders participated in Spain. Like Japan, connections between contemporary and wartime symbols has posed a PR problem for German forces abroad, such as when the German Afghanistan contingent applied a modified DAK palm tree to their vehicles.

So while the immediate postwar period tried to normalize these symbols, their unsavory associations have not been completely erased from public consciousness and they remain a problem for both countries, especially if they send their armed forces abroad.

Consider two factors: one, denazification; two, the way history is taught in America (I'd say "the west" as a whole but I don't have the experience to back that up).

So in the aftermath of World War II, there was a significant effort to remove and destroy everything "Nazi" in Germany to prevent any kind of resurgence of that ideology. But in Japan, the occupation wasn't nearly as stringent. War criminals were convicted just like in Germany, but the Emperor was allowed to stay in place with a rewrite of the Japanese constitution. Japanese civilians were not forced to tour the sites of war crimes or exhume mass graves. The change was not as big an overhaul in Japan.

Second, the treatment of Japan's war crimes in American history classes and how those crimes appear to the typical kid learning about them. For one thing, while the Japanese war crimes could certainly be put on a similar level of evil with the Holocaust, their actions were not quite so systemic. There were plenty of awful massacres, killing of PoWs, etc., but the Japanese-run concentration camps were not typically focused on the mass murder of the people they held. The fact that these war crimes were less systematized means they look just slightly less evil than the Nazis. The horrific rape and slaughter appears more like thoughtless barbarism compared to really intentional, focused genocide. It certainly WAS intentional much of the time, but in a brief overview it's not as easy to clarify that.

Furthermore, American students tend to spend more time learning about Europe just because their history classes tend to be very West-centric, and because the stage immediately after WWII (Cold War) focuses on Europe with Russia vs. USA.

Thus, because it wasn't intentionally removed by the American occupation, and because American students don't learn as much about the war in Asia, the symbol of the Rising Sun isn't associated so strongly with the war crimes.