[impossiblefork]

It's really simple: under the laws of war you can't attack civilians, you can't wrestle them, you can't 'detain' them-- they're off limits.

In cases of crimes they can still be prosecuted, but you can't move them out their country. The crimes have to be in the country in question and under the laws of that existed before the occupation. They can also be prosecuted for war crimes if they commit war crimes.

What the US did that that was problematic wasn't creating prisons outside America-- there can be problems with that, but moving people from, let's say Afghanistan, to a prison in Poland or Romania, or Cuba. That's what's forbidden.

Additionally of course, the problem was that the US was torturing them and keeping this out of the courts, so there were attempts to get people from these bases to the US, so that something could be done legally, but this is a measure that was attempted to find a trick to break crazy situation, not something normal.