It's not a small restriction. You need physical access to a working copy of the disc, and you need an original Wii with gamecube reading capabilities, on top of your Wii U. And you need to move the controllers back and forth. If you have all that, might as well just play it on the original Wii. The rest of us no longer have a working original Wii, or never had one, and the gamecube discs have long been destroyed, lost, or put away in storage somewhere. If someone's original wii laser wore out or their gamecube game was scratched, they should morally still be entitled to play the game. I would not go through all this trouble just to play my remaining discs in good condition. I don't need to play gamecube that much. If I really wanted gamecube, I would just put Dios Mios on my old Wii, because I still have that around. It would be nice to someday have every single version of MarioKart on my Wii U, but that isn't necessary.
Who else has morals like that? Scientists don't hold out because someone might do something illegal with it. Real software engineers don't either. MS Excel doesn't have anti book cooking protection. Google doesn't make sure you can't make drug deals. He's valuing his morality more then the enjoyment of thousands of people. It's his legal right to do that, but we can still call him out on it. Imagine if Bill Gates didn't want anyone to buy Wii Us. So he bought every single Wii U that Nintendo makes, then destroys them. he can do that I guess, but it's still a mean thing to do. On a smaller scale, if I see something good on sale at the store, I usually don't buy out the rest of their stock. I want at least one other next person to be able to get the deal too. If they only have 4 left, I might buy 2, not all 4. It's called being nice and a decent person.