Other news on this has quotes from Tim Cook saying that they'd have to find a way to track purchases on IOS and bill developers so that Apple still gets their cut when using other payment processors. For me, this is way over the line. If they want to charge developers to be on their store, fine. I don't have a problem with that. And charging for using them as a payment processor makes sense. Charging for using Apple…
What Apple are trying to do is to make back the CapEx of developing the OS frameworks developers are relying upon. Those frameworks often make up a non-negligible fraction of an app's USP, i.e. they're partly why the developer is making money.
Apple see whatever slice of market value their OS frameworks added to the developer's product, as morally being their money. Whether there's a legal/contractual framework in place to grab that money or not, they feel like they should be pursuing it.
Compare and contrast: music producers sampling some other IP-holder's work. You'd expect, as a producer, that if half of your song was just sampled from another particular song, then 50% of the revenue of your song would be morally owed to the IP-holder of the other song.
And also, if you had a contract with the IP-holder that paid less than 50% out to them, that'd certainly be legal, but you'd also likely feel that it'd be you getting one up on them — you'd be coming away with more margin than you "deserve", and them with less than they "deserve", given the amount of work each of you put into the final product.
Apple thinks that they "deserve" 10–30% of the value App Store apps capture, because they do 10–30% of the work in making every App Store app (in advance, by building frameworks.) It's not about how the app wants to capture that value, any more than income taxes are about how you make money. They're both about the fact that you did make the money, and the 'infrastructural substrate' that 'invested' in you to help you make that money, wants what it sees as its implicit preferred-shareholder-dividend for that 'investment'.