I don't know why anyone expects they will be allowed to do that. Music is a form of digital IP that had already descended into lawlessness by the time the iPod came along. Movies (because until recently they were too big to download) and eBooks (because until recently there was no convenient way to read them) can still be "controlled" (controlled meaning continue to generate significant revenue for incumbents). Between Kindle and iPad, in 5 years most normal people will have a benchmark in their head that an ebook is "supposed" to cost ~$10 and they will be ok with it. Letting anyone load eBooks onto the iPad risks putting the publishing industry where the music industry is now.
The major difference is that there are many books that people will want to read that are in the public domain. Also, as far as I know, their book-reading competitors (Kindle, Nook) allow un-encumbered ebooks.
