The answer is pretty simple: Japanese were not considered as human. And we all know that numerous governments believe there is no reason to hold back to experimenting on those who aren't human on an equal level or using them for political gain or just posturing, because no one would care.
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To be 100% fair, Japanese had rather low opinion about Americans too, especially during the beginning of the war when the Japanese achieved easy victories everywhere. Many crimes were committed on American POWs (which were also considered only half-human), infuriating American public beyond every measure.
Yamamoto himself allegedly said: "I fear all we have done is to awaken a sleeping giant and fill him with a terrible resolve" and that's exactly what happened in the end, although in 1941. nobody knew how really TERRIBLE things are going to get, but that's unfortunately true for almost every large conflict in human history.