You are right. And we would be justified in returning the disfavor and ignoring the Geneva Convention when our enemies do not abide by it — especially when then neither abide by it nor are signatories to it.
I am all for treating POWs and other with as much respect and decency as possible because after the war these are people we may very well have to live with and animosity is no way to live. That said, I would never risk losing the war, or risk not doing what it takes to win one, just to be thought of a decent or civil. War is not decent. I want my enemies to fear me. War is about killing people and breaking things and neither of those things are of a civil world. I am not the first to say that war should be so unpleasant that belligerents will think twice before waging war.