I can see the argument by analogy to GPL, but I don't think this is about software licensing in practice. It's about control. Apple control how and which apps

I can see the argument by analogy to GPL, but I don't think this is about software licensing in practice. It's about control. Apple control how and which apps get to users, and use this control to impose terms, license or otherwise, that maximize their revenue and other goals. Perhaps you are right that Apple could reengineer the business model to charge software licensing fees rather than payment service fees. They could also make the 30% a publishing fee. There are probably other options too. You can do a lot when you have the power that they have. That said, practicalities matter. If you want to take a cut of a large number of apps' revenue... the place to do it is point-of-sale. Take your cut before app devs get theirs. If payments go to app developers via some other payment system, Apple now needs to chase down a revenue linked licensing fees from 2m apps... that's messy and impractical. They don't know how much they are owed, for one thing.