Is being listed in the App Store a contract? Does it matter that my app was good to go at one point but could be pulled at any time if they introduce a new guideline? How do they justify unilaterally affecting what has to be a billion dollar industry that relies (or what could amount to legal reliance for equitable reasons) on them?
There is a contract, the developer agreement. If you read it, it says that Apple can reject your app for violating it's rules. It also says that Apple has full latitude to reject your app even if it doesn't violate any of the rules because Apple decides it's in their best interest.
Apple can basically pull any app at any time. You can choose not to develop for them, or you could try to sue them (you'd probably have to prove that they did something anti-competitive to get past the contract).
