I think this has to do with the tradition of political theory. For example, Locke holds that in the state of nature, the right to self-preservation takes precedence over the right of others not to be harmed by you, so that if you are starving, and there is someone who has food who does not want to give it to you, you have the right to kill that person if there is no other way of getting his food. When government is created, people cede their right to do violence to others to the state, because the state extends its protection to them as equals. People who have entered into a "contract" with the state, creating it, are citizens. Thus, citizens do not have the same obligation to non-citizens as they do to other citizens, since they never entered into an agreement with these "others" not to do harm to them.

This makes war more palatable, obviously, since in a (non-civil) war, you kill citizens of another country.

As for "collateral damage" by drone aircraft targeting "terrorists", that has to do with another set of issues, the principles of war (and hence the Geneva Conventions). (But it's not even clear that drone aircraft are engaged in war in any other than a metaphorical sense, since they are involved in covert assassinations, not any kind of military actions. Thus their killing even of alleged "terrorists" is illegal under international law, not just that of "civilians".)

Usually one thinks of national law and one's rights as a citizen as more "fundamental" than international law, so there is a sense in which their own government assassinating Americans abroad is more barbaric than the US government's indiscriminate killing of civilians.

But really, all these are fine distinctions. The fact of the matter is that the US has become a rogue state, killing anyone it wants at will, and this is tolerated by its allies, because it has military and other kinds of power over them.

A bomb, H bomb, Minuteman / The names get more attractive / The decisions are made by NATO / The press call it British opinion -- The Three Johns