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

That's iOS, not Android. And even on iOS, Amazon has a video app for viewing videos, though you can't buy or rent directly inside the app:

https://itunes.apple.com/app/amazon-video/id545519333

You can shop via an in-app browser within their Kindle app.