Some of it was cultural differences. The "proper treatment" concept was different between Japan and the Allies. Yes, there were international treaties, and the top brass and intellectuals understood them, but most Japanese soldiers, even the officers, especially towards the end of the war, were conscripted from the countryside and mountain villages. Most of them had never left the country, much less know international law.

Even small differences caused some tension. For example, there is a vegetable called gobo (Arctium lappa) which is widely eaten in Japan. Kinpira Gobo is a favorite of many Japanese and in commonly considered a "household food". There is no Japanese raised person who doesn't know of it's existence, just as there is no American who doesn't know what a hamburger is. However, the gobo is a long, thin, hard, brown vegetable, and looks quite like a stick. The feeding of gobo to POW led to Allied tales that the Japanese were feeding sticks and branches to POWs.

There is no doubt the Japanese treated their POWs just as hard or harder than any other country during the war, I'm not at all contesting that, but there are some mistaken conceptions, which should be corrected.