I mean that's not just an American thing.One of the problems of US politics is that only American bodies count.
You think UK people will give a shit about some guy killing millions on the other side of the Earth when they have to deal with someone killing them by the thousands every day?
If there was a position of president of the Earth that might be relevant but otherwise the death of 'the other' is never considered a bad thing if it is considered at all (and even if there was a rule of Earth, you can be sure that people would turn a blind eye to death caused outside of the sphere).