A good point...
Japanese scientists did some terrible experiments, on humans, in Manchuria during the World War 2...After the war, US did not prosecute most of them because they handed over the data to Americans...
Without a single doubt, too many of the Japanese War Criminals escaped justice after the War. However to me, the more over-riding reasons had more to do with the contemporary geopolitical situation. Manchuria was quickly occupied by the Soviet Union and then handed over to Mao's forces. Getting on-site inspections was not on the table. Add to that the very real danger from the Japanese Communist Party grabbing these issues as unfair prosecutions while the American Occupation (MacArthur) was trying to disarm Japan. Not saying you are wrong but just pulling in another justification for this injustice!