The Geneva convention covers the way you show POWs on TV, not just having them on the screen. It was done to attempt at preventing them becoming pawns in the politics of war. Imagine if there was no rules, the Iraqis could turn into flat out hostage holders, just having one of the POWs read a message "this war is unjust and we didn't want any part of it, and now if you don't stop they are saying they will kill us one at a time for each 24 hours the bombing continues". That is exactly the reason why you are allowed to show the existence of POWs, but not have them make statements of any kind.