take a perusal through this:
https://en.wikipedia.org/wiki/Japanese_war_crimes

several of these attacks resulted in hundreds of thousands of dead, at one time.

they were horrific - and they had to be stopped.

so, if the question is "why did americans accept this?", the answer is to look at what the japanese did, first. and, i can't help but think that anybody who seriously knows the history of japanese war crimes in the second world war would be less than empathetic about what happened to them, in the end.

the easy answer is that if you don't want to get nuked, don't try to conquer the world and slaughter hundreds of thousands of people at a time, repeatedly, while you're at it.