The question of whether or not the government should be able to use AI for targeting without the involvement of humans is a wartime question, since that is the only time the military should be killing people.

Under such a scenario, requisition applies, and so all of this talk is moot.

The fact that the military is killing people without a declaration of war is the problem, and that's where energy and effort should be directed.

Edit:

There's a yet larger question on whether any legal constraints on the military's use of technology even makes sense at all, since any safeguards will be quickly yielded if a real enemy presents itself. As a course of natural law, no society will willingly handicap its means of defense against an external threat.

It follows then that the only time these ethical concerns apply is when we are the aggressor, which we almost always are. It's the aggression that we should be limiting, not the technology.