A large number of products are being sold with some docs that has a link to their own merchandise store and promote them? Apple doesn't have to tell about the competitions on their app store, but they should allow each app whatever they want to do.
So you think it's OK that Walmart doesn't want to sell a product that says, "Hey, don't buy this from Walmart", but you think it's wrong for Walmart not to want to sell a product that links to a website, where the website says, "Hey, don't buy any more of our stuff from Walmart"?
Nintendo will encourage you to buy from their online store, competing with Wal-Mart selling physical media (and maybe digital codes too on their own store?).
So to be clear: if Apple printed on every iPhone box, "This phone is 30% cheaper on Apple.com", you feel that Amazon, Walmart, Best Buy, %LOCAL_TECH_STORE%, etc. should be legally obligated to stock those iPhones?
A better question is, why aren't iphones cheaper at apple stores?