It all boils down to the type of war one is in, the perspective of the populace being killed, and who is doing the killing.

Most populaces don't like being killed by some invader.
Most populaces don't like being killed by their own government.
Most populaces don't like being killed by some other government in support of their own goverment.

Most populaces will tolerate being killed by their government or those who help their government in order to remove an invader or some greater evil (as measured from their perspective, not anyone elses).

Often populaces must be targeted directly as a supporting effort to breaking the will of a nation waging a state-based war against your state. (#### happens, you started it, and you must KNOW you are defeated when this is truly over so you don't start it again as soon as you can breed, raise and train a new army).

There is no situation that the US is currently involved in where civilian casualites (though sadly unavoidable) are anything but detrimental to the ends we are seeking. Sometimes this means not taking a shot, as the potential tactical gain just isn't worth the potential strategic loss.