I mean I agree with you (US submarine campaign had already completely cut Japan off), but I think it's pretty obvious that the reason it's "controversial" is the fact that Japan was run by a fascist imperial government that was responsible for the deaths of millions of civilians in China and Southeast Asia. I think it's pretty hard to judge the restraint shown when the Axis powers were the ones who started attacks on population centers with events like the Rape of Nanking and the Bombing of London. Most of the the point of the UN was to ensure it never happened again, which it hasn't.