I disagree with your analogy. iOS is not a store. The App Store is a store. I'd instead make this analogy: The owner of a bookstore is also the mayor of the town. He enacts a law (via declaration) requiring all bookstores to accept his bookstore's credit cards (of which he gets 30% of the sell price).
Luck for Apple they aren't dominant in ebooks. If they do the same thing to iTunes competitors (and it looks like they might), they could be in real trouble with regulators.
Speaking of bad analogies, iOS is not a town that Apple just happened to get elected mayor of. It's more like Disney World and Apple is Disney. Good luck starting your business inside Disney World without living by their rules.