Frankly anyone who says this with a straight face is being absurd. Apple's commission isn't for "payment processing", promotion, any particular API, or service they provide on their platform. That they do all of those things makes it an attractive place to develop for and sell your product, but it isn't the reason why they get to charge 30%.
They charge 30% because they've amassed 1 billion extremely valuable users who are willing to spend money on apps in their marketplace.