Not entirely. It isn't clear that the US government can start killing Americans when they take weekend trips to Toronto without violating the Due Process clause.
But all that would do would be cause an international incident and expose those who carry it out to threat of prosecution of not granted diplomatic immunity.
The question is whether US law prevents it. THere are some cases to suggest that the bill of rights still governs to at least some extent when a US citizen is acted against by the US government in a foreign county.
