The Japanese were not at all averse to attacking civilian targets. For example, their bombing of Chongqing (the Nationalist capital after their retreat to the interior after the loss of Nanjing (their capital) and Shanghai) killed over 10,000 civilians long before the attack on Pearl Harbor. While this is probably the best-known of the Japanese bombing campaigns against civilian targets in China, there were other targets (e.g., Xining).

To such conventional bombing can be added biological warfare against civilian targets, and "field tests" of biological agents on Japanese-controlled Chinese cities. Reliable estimates of Chinese civilian deaths from these are difficult to make, but the better estimates tend to vary from 200,000 to 600,000.

To these we can add various massacres. The most famous is the Nanjing Massacre, but there were others, too (including outside China). There were also selective killings of civilians, such as ethnic Chinese in Malaya and Singapore, Hui Moslems in China, etc.

Overall, the majority of the approximately 8,000,0000 Chinese civilians who died during the war due to direct military action were killed by the Japanese. While fewer, there were also many deaths in the others parts of Asia where Japan fought. (This excludes famine and disease deaths, including deaths due to biological warfare. It should be noted that many famine deaths (e.g., 1-2 million in northern Vietnam) were due to Japanese confiscation of food.)

Given such a record of attacks on civilians, why didn't the Japanese attack the USA similarly? The answer is simple: lack of opportunity. US cities were out of reach of Japanese air power - the attack on Pearl Harbor was a major effort, and the limit of what the Japanese could do, and depended on surprise. The Japanese did what they could. In early and late September 1942, the Japanese bombed Oregon with a submarine-carried floatplane to try to start forest fires (the "Lookout air raids"). Their balloon-bombing campaign of 1944 and 1945 used high-altitude balloons driven by the jet stream to drop bombs on the US (with, of course, negligible accuracy - one of the goals was, again, to start forest fires). It was intended to also use these balloons for biological warfare against the US, but the war ended before this could be put into action.

Early in the war, the US was in a similar position - there were no Japanese civilian targets that could be attacked, with the exception of special surprise missions such as the Doolittle Raid. Later in the war, with the B-29 and airbases within range of Japan, Japanese cities could be attacked, and were. However, Japanese military targets were not left alone. US forces continued to attack Japanese military targets - bases and shipping - whenever they could.

One final point: many civilian targets are also military targets. When attacked by imprecise weapons such as heavy bombers, many civilians are killed. Sometimes, the attacks are made with a specific short-term military goal in mind, such as to put a major hub in the military transport network out of action to support an offensive, and can be reasonably described as attacks on military targets. One example is the US raids on Wuhan in China, in December 1944. At that time, Wuhan was a major Japanese military base. The bombing destroyed much of the city. The US aim was certainly not to kill 20,000 Chinese civilians, yet it happened.