In the March 14, 2003 memo, Yoo says the Constitution was not in play with regard to the interrogations because the Fifth Amendment (which provides for due process of law) and the Eighth Amendment (which prevents the government from employing cruel and usual punishment) does "not extend to alien enemy combatants held abroad.":
The memo goes on to explain that federal criminal statutes regarding assault and other crimes against the body don't apply to authorized military interrogations overseas and that statutes that do apply to the conduct of U.S. officials abroad pertaining to war crimes and torture establish a limited obligation on the part of interrogators to refrain from bodily harm.
Source: The Swamp
And it's a funny thought, because I'm looking at the 5th Amendment of the United States Constitution:
No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual service in time of War or public danger; nor shall any person be subject for the same offence to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation.
No person. Not citizen, not voting member, not white or black or foreign or domestic. Person. It's clear to me that the Founders wanted it that way. Can you imagine how long it would take for something to declare "Well, you aren't a citizen - I strip you of that definition." Something that I believe was attempted in the case of John Walker (correct me if I'm wrong), but some argued that he couldn't claim to be a citizen any more. Sure, he was a douche bag - but he still had certain rights we had to follow.
Later on, the Geneva convention was created to afford similar rights to all people. Oh, but I guess if you're not a soldier for a country, we don't have to follow that, either. How do you define a real soldier? By however we say. They're in another country - that means we don't have to give them habeas corpus.
I've seen people say "Oh, well, it wasn't on US soil, so it doesn't matter." I'd argue the same thing: doesn't matter. Again, look at Amendment 5 and 8 - they don't say "US soil", or "US area". Think of the implications:
"Well, we arrested them in Berlin. So we don't have to give them a Miranda and we can hit them with a pipe, then use that evidence in trial."
Or: "We arrested them in the US, then flew them to Syria, where they were tortured and confessed. But we can use that confession since it wasn't on US soil".
The Constitution laid it very clear. The restrictions on government power aren't to be ignored when convenient. Or put aside in times of war because you're "protecting the homeland." Or ignored because you want to redefine what a person is. These are the rules. They apply to all people. At all times. In all places.
Because otherwise, they don't really apply at all.