I like the similarity, but you have to make it more honest:

You're a writer, but don't have a good typewriter. Apple is willing to lend you one for free. The only issue is that if you then want to sell your book, you have to sell it through Apple's store. If you don't sell a single copy, Apple won't charge you for the use of the typewriter. If you sell a lot of copies, Apple gets a cut, both for the book being in their store (publicity) and the use of their typewriter. You're still allowed to use different typewriters, and you can even sell books written on a different typewriter in Apple's store, though they'll still take a cut.

Doesn't sound terribly unreasonable to me