Earlier quoted context omitted.
That something is written on a contract does not means that is enforceable. If not I will create a contract that says you are now the president and you will still not be.
The contract developers have with Apple involves use of Apple's property. A contract may not be enforceable, but only if it contradicts the law. In this instance, I don't see how.
I suppose if you don't like it you can go buy an Android phone and just give away all your data in addition to the 30%.
Also worth considering: How much of Apple's intellectual property here could exist without them harvesting decades of free/open software development history unpaid? And now they're complaining that someone used their SDK after paying the license fees?