You're right, in that there's no reason to assume the bombs were entirely decisive by themselves. The truth is that from the target's POV, there was nothing particularly special or interesting about the atomic bombs of the day, except that they were dropped from a single plane.
So no, they wouldn't be considered war crimes, any more than the equally-destructive firebombing of Tokyo and Dresden would be considered war crimes. Meaning, of course they would be considered war crimes, but only if the victims had won the war. That's the idea behind war. War is about doing the worst stuff you can do to the other guys, then doing whatever you can to claim the moral high ground afterward. So it's best avoided when possible.
Szilard was a great guy, and in fact he was behind the original missive to FDR that kicked the program into gear. It's as impossible -- and as inappropriate -- for us to judge him and his motivations as it is for us to second-guess Truman's decision to drop the bombs. However, he's all wet with that particular argument. Unlike Germany there was never any question that the Allied side would win the war, bomb or no bomb. The question was, what would be the cost, and who should pay that cost. I'm fine with Japan paying it. They would certainly have done the same to us, and they would certainly have skipped the subsequent navel-gazing.
By the way, it's easy to argue that the 'slight geopolitical edge' that the Bomb gave us over the USSR saved millions of lives in the future. For instance, it's far from clear that North Korea wouldn't be better off today if MacArthur had been allowed to have his way.
Imagine that the Russians had either somehow beaten us to the Bomb, or had invaded Japan in the absence of our ability to deter them. Given a choice between suffering Hiroshima and Nagasaki at our hands, and suffering a half-century of Communist rule, do you really think Japan would be better off in the latter scenario?
