There's an article here on Apple's tactics re the iPad. Most interesting is this: "Amazon, Barnes & Noble Inc. and Google have all created applications for the iPhone and iPad that allow readers to access books they've bought through the companies' websites. But Apple now says that those booksellers must also sell their books through Apple's store or risk getting their applications booted from the iPad. Apple is also requiring that any e-books a company sells outside of Apple's store must be the same price — or higher." Therefore Amazon presumably can't up its prices on its iPad app to compensate for the lost 30%. They would just have to eat the loss, which I doubt they are willing to do.
An analyst quoted in the article says, "I honestly see this as a big mistake... Apple invited its competitors to be its partners and is now changing the rules on them."