Item frames case their position off block position even though they're entities (and thus should use the decimal positions). This limit the use of item frame usage with command blocks and plugins. If we didn't have this limit people would be able to create moving builtin boards, menus that float in front of them, ect.... This would also open up the ability to edit the item frame rotation and thus make diagonal things, witch would be a very powerful tool for custom map makers and the likes.