Probably because they bombed even more cities and killed even more people shortly before. Not to mention they’ve bombed even more cities and killed even more people in Korea few years later, and then even more at Vietnam, Laos, Cambodia.

So they don’t even have to rationalize aerial mass murder, since it’s just completely normal way of waging war for them - they just adress the atomics, which is not really very big step up from the firestorm in Tokyo, destruction of literally everything in North Korea or the poisoning of Vietnam.

And of course being libs they could not spare us their hypocrisy - they do all of this while being shocked by the events like bombing of Guernica or London. Imagine what would they say if any socialist country actually done any strategic bombing…