Not just the Japanese. I knew an old chap who told me that his father was a POW of the Germans during WW1. Towards the end of the war Germany was very short of food and POWs were basically starving. This chap's father told him that if one of their number died it would be thought ok to say take off a leg to cook and eat before the body was taken away.
No different from organ donation to save life, if you think about it.