Long story short, the GPL requires you to grant a license to run the application for any purpose. You can also not add additional constraints to this. The App Store's terms and conditions say that you may only offer the software to be used under the usage terms that Apple gives. These two things are in conflict. Apple generally does not approve apps with a GPL license and most people think it's because of this reason (as far as I know they haven't ever made a comment about it). A few apps have apparently made it through, it seems this has been an error on Apple's part.