The top story at CBSNews.com this morning bears the title, "Donald Trump suggests trying Americans at Guantanamo Bay." It is a headline intended to shock the reader -- after all, since the administration of George W. Bush it has been American policy that most of the U.S. Constitution does not apply to actions taken by the federal government in its overseas territories like Guantanamo Bay. [Don't worry, though: the part that makes the American president commander-in-chief of the American military applies everywhere and at all times, and unlike the Bill of Rights, there is never any talk of balancing this against any other consideration.] This ends up meaning that "detainees", an Orwellian term if ever there was one, have no status and no rights whatsoever; they are essentially considered non-human, which is very different from how we treat even convicted criminals, let alone prisoners of war. Guantanamo Bay has become America's oubliette, the place in which we throw people to forget about them. This has been eased somewhat by the judicial branch, but only over the objections of the other two branches of American government.
So it is actually quite easy to see why Americans would be shocked at the idea of U.S. citizens being subjected to such treatment by the American government. The real question is why more Americans aren't shocked that the U.S. government is allowed to do this to anyone.