Apple doesn’t disallow the use of private APIs out of spite; they disallow it because their private APIs are not fully baked.
It should be obvious, but: Apple’s intentions are utterly unimportant in this matter. Using private APIs for system-level applications is one thing, but iBooks is in the App Store. iBooks uses private, Apple-only APIs that are not available to other developers. iBooks directly competes with any number of other applications that sells things to read. iBooks is using an unfair advantage given to it by Apple’s engineers. There’s nothing ok about any of that.
Adding to this, there is the potentially coming storm of a United States .vs. Microsoft situation regarding the iTunes Music Store. There are other music stores out there, after all, and Apple has not put them all on equal footing. If using special features that no other developers have access to isn’t an anti-competitive practice, I’m not sure what else qualifies.
The more iPads, iPhones and Macs that Apple sells, the closer they’re coming to an antitrust showdown.