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.
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.