The reason why Guantanamo is controversial is not because it violates international law, but because it sidesteps international law. Because terrorists are a form of combattant not directly covered by the GC, technically the U.S. is not violating any laws.
However,
in principle, it is questionable since the U.S.
did sign the Geneva Convention under the premise that unfair detention and questionable interrogation methods are not valid under a just international system. It's basically a moral/ethical debate considering the U.S. signed that Convention in good faith. The U.S. has lost face in the matter because it has, in the past, tried to portray itself as a nation of justice and liberty, and yet it has demonstrated that it won't play fairly according to its own image if a piece of paper is not telling it to. So basically, my issue is that the U.S. is operating under a hypocrisy. If you are actually a nation that detains people unfairly and sidesteps international law, then stop pretending you are so goody goody, when you are just like every other country.
It's not that the U.S. has the "right" to create Guantanamo, it's that it can. The Bush Administration cherry picked technicalities that casted reasonable doubt on the application of the GC to terrorist detainees. The key point that matters is that this reasonable doubt was never heard in an international court, but was one government's say-so. As one of the world's hegemonic powers, its questionable acts are untouchable. If a smaller, less powerful nation did something like that, it would likely be attacked or stripped of economic resources, or dragged in front of a world court. Once again, the reasons for this political mess are power related.
Also, not all conditions at Guantanamo were in violation of the GC anyway. For instance, Bush said in
this White House press release that: "the Geneva Convention will apply to the Taliban detainees, but not to the al Qaeda international terrorists." Since the Taliban was the Afghani government, they are considered detainees of a sovereign State and thus the GC applies. Terrorists, however, are transnational actors and therefore, according to Bush, detaining them as actors of sovereign states is not an applicable policy since terrorist networks operate globally. This means the Bush admin can do whatever it wants to those detainees, including water boarding. I would, however, have preferred to let an international court decide on that one. However, that would never happen, due to the level of power the U.S. has.
That said, the Bush Admin has already made several statements about the proper treatment of the prisoners (i.e. proper meals, access to religious rites, etc). The issue of waterboarding is something else, however, and I don't agree that it's a fair practice.
I mostly disagree with the existence of Guantanamo in principle, since it contradicts the image the U.S. tries to project of its own fair and just nature. Now we know that that image is just a guise and the U.S. is a self-interested actor just like anyone else. And yet, some people still believe the propaganda. The next time the U.S. points the finger at anyone else regarding unfair detention practices or even torture, it will have zero credibility. I believe this is the main reason why Guantanamo is now slated for shutdown, as it tarnishes U.S. reputation further.