The fact that an operation may target a U.S. citizen does alter this conclusion. As explained above, the Supreme Court has held that the military may constitutionally use force against a U.S. citizen who is a part of enemy forces. Similarly, under the Constitution and the inherent right to national self-defense recognized in international law, the President may authorize a use of force against a U.S. citizen who is a member of al-Qa'ida or its associated forces and who poses an imminent threat of violent attack against the United States.
(citations omitted)
It further goes on to discuss how it wouldn't violate assassination ban and must be conducted in accordance with the laws or war.
Soooo... where is the limitation on borders? Simply the prohibition on the use of US military assets within the US, would be my guess. But there's already bans on targeted assassinations, and the WH carefully maneuvers around that problem, so why wouldn't we expect more of the same?
ETA: I've kind of moved the discussion into the slippery slope arena, but there's still a pretty fundamental problem here. And that is this: two US citizens could be engaging in the exact same activities, but one is in a Texas border town and the other is 100 miles across the border into Mexico. One has to be arrested and tried before being executed, but the other can be killed after we say "Hey Mexico do you mind if we send a SEAL team into your territory and capture one of our peeps (pleasesaynopleasesayno)? You do mind? Aw shucks, send in the drones then." Or "Hey Mr. Joint Chiefs, do you think that some of our boys would get killed if we sent them to get John Q. Terrorist? You do? Aw shucks, send in the drones then." That's okay with some of you?