Whether or not to play in Apple's iOS garden is a business decision companies like Amazon or B&N will have to make. There's no reason for them to offer iOS versions of the e-readers. Oh, except for the large customer base. If that customer base is big enough I'm sure Amazon and B&N and others will agree to Apple's rules. 70% revenue for a customer pool of millions of iPhone and iPad users is better than 100% revenue for zero of them.
Apple is offering others the ability to take advantage of their platform. How many Nook books can you buy from B&N on the Kindle, or Apple iBooks on the Nook? None. Apple is creating a place where Amazon and B&N will be able to compete with iBooks on price using the same e-reader. Neither Amazon nor B&N open their gardens to competitors.