You fundamentally misunderstand the rationale behind the conventions, and particularly the parts of them that apply specifically in conflicts that are not conflicts between state parties (e.g., Common Article 3.)

Anyhow, there is also the Convention Against Torture isn't even related to conflict or POWs, but applies universally, and obliges parties to it (including the US) to prosecute (not merely avoid engaging in) torture, whoever does it, wherever they do it, and in whatever context they do it. Torturers are hostis humani generis, the common enemy of all mankind.