I am confused on this issue. I'm not sure I even understand the question. Here is my thinking, so please comment:
Suppose the US was at war with Country X. Men with guns attacked a US military base in Country X. The US troops fire back, killing the forces of Country X. But aha! One of the enemy was actually a US citizen! So does that mean the US troops cannot shoot at that one person?
Suppose the US was at war with Country X. Country X had terrorists bombing buildings in Country X. The US send drones to shoot at the men who have been bombing buildings. But aha! One of the bombers was actually a US citizen! So does that mean the US troops cannot send drones to shoot at that one bomber?
I'm unclear why the citizenship of the person has anything to do with the military action used against them. I am also unclear why the method used to fire upon the person changes anything either. Would it make a difference if the person was a US citizen because they were born here but left 2 days after birth? What if they were a naturalized citizen who was a resident for more than 7 years?
Why is it okay to target non-US citizens with drones, but not US citizens? Why is it okay to shoot them, but not with drones?