It's hardly buried: it's mentioned in the description of the app, it's mentioned inside the app when you "publish", it's mentioned in the help file for the app, and it appears, in boldface letters, as an "IMPORTANT NOTE", at the _beginning_ of the EULA.
Also worth mentioning: the economics of K–12 textbook publishing are very different than mass-market publishing: essentially 100% of sales are volume purchases by educational institutions who are already accustomed to distributing materials to students.
This is Apple's proposed alternative to publishers bundling interactive content with expensive textbooks (probably "free" if available on the App Store), and the value proposition is wide exposure, lower infrastructure costs, and no used textbook market to undercut profits in exchange for "a la carte" distribution of individual "unbundled" books and a unit price cap.
In other words, it really is an attempt to apply much of the "iTunes business model" to textbook publishing, and it'll be quite interesting to see how it works out.
It's still a trap. When you copy and paste your content into the iBooks software, you instantly hand over all the distribution rights, as part of the terms of an EULA of a consumer product. Sure, this is the kind of thing that the record industry does if you sign your band over to them, but this is a consumer product we're talking about. This is far beyond what a consumer should expect from such software. If Apple wants exclusive distribution deals, it should make people sign for it so that both parties are completely aware of what they're getting into.
And the iTunes business model was never about exclusivity. You can buy most of the music on iTunes elsewhere. (There is some exclusive content, obviously, but it is the exception, not the rule.) And, most importantly, the deals negotiated to get music on iTunes are done the old fashioned way: as a mutual, signed agreement between Apple and the labels in question.
I don't think you understand -- they're only restricting what you can do with the output of the program, not your content itself. You're free to copy/paste it into any other tool and distribute the output of that tool.