Again, how to explain logically why the action is disabled in a location which, other than the possibility of being game breaking, should work story-wise would be difficult. Too many games give you cool features, and then limit randomly them since cool features have a tendency of breaking the game somehow without limitations. They didn't want to give you an amazing feature that must be limited in a seemingly random way, and even more so have to figure out all the ways, places, etc. to limit it.