The bigger issue is that some subscription services have less than a 30% profit margin already. This policy would mean losing money on every iPhone user that signs up through Apple. The obvious solution is to raise prices, but Apple's policy is that the iTunes price cannot be higher than the price available anywhere else. So prices go up for everybody, whether or not they are an Apple customer.
For example, Amazon gives publishers ~70% of the purchase price of a book. If they have to give the 30% to Apple, that leaves no profit for Amazon. Apple would end up with a huge ebook store with none of the overhead of actually negotiating with publishers (the guidelines require that Amazon provide the content for sale through the app store). When it becomes impossible to profit on Apple platforms, I'd expect Amazon to put their money behind Android and any other platforms.