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Like, say, status of force agreements?
I'm really surprised you didn't come up with the rest of the Yoo and Goldsmith arguments: If a person is not a national to the country of the conflict, they can be deported, and therefore shipped to Guantanamo without any extradition, if a civilian has been arrested and not yet charged they can be deported since they are not yet a GC4 prisoner, and on and on and on. Cruel and unusual punishment doesn't apply before a prisoner is convicted of a crime, and all the rest. Importing definitions of severe pain that are supposed to govern when 911 gets called for Medicare patients for use in torture statutes.
Your arguments rely on a group of people hypothetically acceding to a treaty they are forbidden to accede to, and punishing them for failing to do so by treating them inhumanely. There is nothing in the Geneva Conventions that permits that, creative interpretation doesn't change that. If you want confirmation for your arguments, look up John Yoo's documents which relieve the U.S. of responsibility for Geneva vis-à-vis al Qaeda. They look a lot like what you write, and they are considered bad law by IHL experts. Yoo wrote the documents in January 2002, they are accessible on the web. So are Goldsmith's GC4 thingies.
On top of which, many of the al Qaeda arrests were made in a country that was not party to any conflict, and they weren't made on the battlefield. How do you determine that at the time of arrest they were not obeying the Geneva Conventions, or that they were military or civilian? Or even, in some cases, that they were not citizens of the country of conflict? If you arrest a Pakistani in Pakistan when your conflict is in Afghanistan, I'm sorry but that person doesn't have to display any arms openly, any sign, or be an Afghani, because you simply haven't picked him up in armed conflict in the war zone. Geneva doesn't allow you to speculate on what he might have done had he been on a battlefield, with respect to complying, based on his affiliation. But you round up Afghan goatherds for bounty in Afghanistan and transport them to Guantanamo, and you may in fact have committed a crime, since civilian deportation by occupiers is not allowed and you have no proof these people were belligerents. And your arguments would work, if they were valid, for rounding up all NGO/NPOs who are not Red Cross. Do you really think you can imprison and abuse CRS workers? The Vatican didn't sign Geneva either.