Apple should have a different model for re-sellers
If Apple is going to require this, they should (IMHO) have a reduced revenue cut for themselves. It is one thing for a developer (originator of content) to give up 30% (of 100%), versus a re-seller of content like Amazon to give up 30% (of ?) of their selling price/retail, which is already a percentage cut from the content owner/developer.
Apple, of course, can do whatever they want (this is not a negative comment - they are running a business - note: does Amazon allow externally bought content on their Kindle... No). However, Apple, 1) has always been more about selling the hardware (the content drives hardware sales); and 2) has been end-user focused as far as giving the best possible experience of the content. Amazon, on the other hand, is about selling books - the Kindle is a vehicle for that, and so is the iPad.
It will be interesting to see how these two different business models may or may not cooperate. Maybe, Apple is looking at making book sales a bigger revenue stream in the future. iBooks still doesn't have the depth of content as Amazon (with Amazon locking up what they can - what about an article about "bad Amazon" trying to lock up book sales - make it a "closed system" for themselves). iBooks, by the way, is also really slooooooow on an iPhone 3g (at least mine) - to the point of being unusable.