I think you're confusing Google with Apple. The Play Store's policies are much more reasonable, and allow third party payment processors for purchasing digital items that can be consumed outside of the app itself. In fact, here's a third party video store on the Play Store [1], and here's Amazon's own Prime Music app [2].
[1] https://play.google.com/store/apps/details?id=air.com.vudu.a... [2] https://play.google.com/store/apps/details?id=com.amazon.mp3