I think it might have something more to do with the fact that the Germans did it to their own citizens, whereas the Japanese were being cruel to enemy soldiers. Some people may consider that difference significant. barfo
One set of grandparents were Russians who emigrated to China after the Russian Revolution. My grandfather was a naval officer under Czar Nicholas who later served in the Pacific. They left Shanghai after the Japanese invaded. The stories they told about what the Japanese did to the "genetically inferior" Chinese were horrifying. If you can stomach it, read a book titled "The Rape of Nanking".