Apple's own applications have been using APIs and have had capabilities unavailable to third parties since day one. The only difference with iBooks is that the app is distributed through the store, but that is completely orthogonal to the issue of private APIs. I cannot make myself get outraged about this.
I wouldn't say I'm outraged over it, but it seemed like a nice system to have the built-in apps (which can't be competed with since Apple explicitly disallows apps that duplicate built-in functionality) have free reign to do whatever they wanted but have Apple's own App Store apps, which compete with 3rd party apps, do so on a level playing field.
There's a fairly substantial difference, in that all Apple apps that used private APIs came installed with the device. They weren't competing for a share of the app store market.
Apple is now directly competing for the same dollars as everyone else on the app store, but they're doing so using tools that nobody else is allowed to use.
