I think it mainly has to do with games that are really online-focussed like Halo. Becouse taking screenshots pauses the game for a fraction of a second, this would create lag in singleplayer. Maybe there is a work around?
Well the thing its the game giving you problems,also I have noticed if you press the xbox guide button on the controller then you can take a screenshot just fine.Maybe using the latest sdk(august build) might be of help.