It has always been my understanding that MacArthur allowed the Nips to get by mostly on a wink and a nod, that the atrocities committed in POW camps weren't part of the policy or orders of the Imperial Japanese Army high command. General Homma was tried and executed for Bataan, but there was no deep-dive follow up on the actual perpetrators like there was for the Nazis, chasing down every corporal and sergeant who carried out the murders.
It took the fucking Soviets to prosecute Unit 731, for God's sake.