Likely the intent and goals of our regimes, as well as the fact that the US doesn't explicitly kill civilians for political goals (we don't go around killing folks in Czech-Republic because we want to control their territory).

Yes, we have attacked civilians (firebombing Tokyo, the atomic bombings of Hiroshima and Nagasaki) but the United States is one of the few nations in the world that hasn't had a socio-political revolution (i.e. Bolsheviks in Russia, for example) and has had consistency throughout its lifetime as far as relative government stability. I think that because of this, we haven't had a dictatorial or totalitarian government that enforced its will upon the region through terror and violence. We haven't had a Stalin, or a Mao. In fact, we've had more weak leaders than strong figures that are easily remembered (William Henry Harrison, anyone?)

Not saying that is why, I'm just reasoning. I mean, I know I certainly wouldn't put Truman on the same level as Stalin in this case (though Christ may disagree and Harry S. might be pitching brimstone in Hell right now).