For the same reason it's not legal to kill Mexicans crossing the border illegally. The standards apply to us, and that's all that matters. Now, these folks don't get full constitutional rights (for example, they are not required to be Mirandized, nor will they ever be).
The higher theory is Natural Law, which is the same school of thought that allows soldiers (who sign away almost all of their rights to serve in the military) to question and in cases, to disobey, orders given from commanders to shoot into a crowd of women and children.
Onto my other point on Cheney - I forget who wrote this:
Get used to this sort of thing. Cheney needs to spread this sort of nonsense. Every day that the Obama administration rolls back his legacy and the U.S. isn't attacked is another day in which Cheney's contentions that the U.S. needed to embrace torture, preventive war and illegal surveillance in order to be safe is debunked. He has little choice but to spread the counternarrative that we're actually just another day closer to another attack. That isn't surprising. Cheney is what he is. What's worth watching is how many media organizations, in order to secure the Newsmaker Interview, let him get away with it unchallenged.