There's the possibility that Apple simply wouldn't allow the player at all, and if they did, Apple would take a 30% cut of all in-app purchases. Amazon may not

There's the possibility that Apple simply wouldn't allow the player at all, and if they did, Apple would take a 30% cut of all in-app purchases. Amazon may not be able to sell music with Apple taking a 30% cut, since they likely have contractual obligations to pay a certain percentage for their music.