Right but there's no material difference between the two. You just have a choice between which master to serve. Amazon's platform isn't more open, they've just allowed other music services to operate on their platform.
Amazon approval is basically a rubber stamp. Can't say the same for Apple's. I'm also not aware of Amazon denying anything for competitive purposes. Amazon is happy to let Spotify on their platform, despite having a music service of their own, for instance.
https://developer.amazon.com/alexa-skills-kit