I'd say that Apple's 30% is a commission based on the sales and service they offer, and yes it's worth it...especially given the market position and the fact that there's verification of IP ownership, etc. But in the end the app itself belongs to the creator, just as the music belongs to the artist (or label, depending on the deal.)
Seems a bit like perspective, but it goes deeper: if the music belongs to the artist who created it then they should have the right to pull it from a service and expect reasonable effort to keep it out of the service — YouTube's content ID does a commendable job. (Though it is abused by some of the majors...but the algorithm itself is really solid.)
I've develop a few iOS apps. The money is Apple's until they pay me. I'm entitled to that money under the payment schedule we agreed upon, but Apple can do whatever they want with it in the meantime. I'm sure they do make use of it.
Wrong. Musicians can choose to distribute the music themselves or through a variety other third parties. iOS developers have no other meaningful choice.