Try reading this: http://www.drinkerbiddle.com/files/ftpupload/PORTAL/Kunz_Art...

This is an article the legal reasoning behind why the iBookstore MFN clause was an antitrust violation - it was designed to impose financial penalties on publishers that did not get all of their retailers to switch to an agency pricing model.

This is similar to an earlier case about Toys 'R Us. They got a group of toy manufacturers to agree to restrict their toy sales to warehouse clubs, so long all the manufactures in the group agreed to do so.