It's not just about legality it's about an agreement between the creator and the buyer. Anyone can break the rules of the game and people do all the time but we are dealing in areas of trust... and you're right there is little way to give any of this teeth.
We trust the artist will stay true to the game and not simply create more and more tokens of the same image after saying there would only be X editions.
We trust the buyer to do a few things... and sadly it's hard to enforce either except to point out a breaking of the rules of the game