there can be no question but that they are legally Prisoners of War, and that America has very clear duties towards them under the Convention.
I disagree that there can be no question. When one reads the text you have linked to one sees that article 4 defines who is prisoner of war (and by implication, who is not) in the sense of the convention. I have no doubts that the Taliban belong to this category, I have much more doubts that Al Qaeda fighters fulfill all requirements of Article 4, (2), (a) to (d). It all depends how you understand fighting on behalf in your post. The US government here has not such a weak position as some like to think. At least some of the prisoners they keep outside of Afghanistan may not be prisoners of war according to the Convention.

The UN charter of human rights, however, is not restricted to a subgroup of humans. That charter in my eyes is violated by the US government's treatment (no process, not even a timeline for such a proceeding) of the prisoners in Guantanamo and elsewhere.

Wolfgang