I think of it as:
"Hey, we bought the game, so we have the right to do what the heck we want with it."
You dont buy a cheeseburger, and the person selling it cant say you cant put ketchup on it.
You aren't buying a cheeseburger with copyright. The game is copyrighted, so technically nintendo still owns everything inside the disk. By sharing anything inside the disk modified or not is illegal. But who cares, i don't think nintendo cares all too much, its keeping their game alive.