In Apple's mind, iTunes and iBooks are competing services to Neflix, Hulu, Rhapsody, B&N eReader, Kindle eReader, except that iTunes and iBooks have less features or less content or worse experience or no streaming. They are all ways of providing access to view/read/listen to digital media from content producers. Right now, you may not view iTunes and iBooks as competing services because both are available on the same platform and provide different ways of accessing digital media.

Apple currently has to allow these competing services to be made available through their platform, either to make the platform more appealing to end consumers or because of worries of anti-monopolistic concerns or because Apple isn't comfortable with denying these.