What's wrong with treating the terrorist prisoners exactly the same as the terrorists treat prisoners?
To paraphrase, the Geneva convention is not a suicide pact. Name one enemy of the US that has abided by it.
The Germans in WWII. There were some exceptions, to be sure -- the Battle of the Bulge incident and, on occasion, the Waffen SS. But, for the most part, the Germans scrupulously followed the Conventions with their US, British and Dominion prisoners.
Neither side followed the Geneva Convention on the Eastern Front, of course. But I don't believe the Soviets were signatories.