Apple is providing free tools for authors to create books. If you want to give away your book for free, you can do that. For example, if a teacher makes an iBook for students, they can give it to them at no cost and Apple doesn’t care.
If, however, you create an iBook using Apple’s tools and you want to sell it, then you have to use the iBookstore and give Apple its cut.
That sounds fair to me. Use Apple’s tools, sell your product, and give Apple the money it deserves for providing you with a way to make and sell a product.