This is not directly comparable to the app store fees that apple/google make. The cast API is free to use AFAIK, although still within google's ecosystem. Amazo

This is not directly comparable to the app store fees that apple/google make. The cast API is free to use AFAIK, although still within google's ecosystem. Amazon's content app itself is available on google's store. The decision to not support the cast API would be due to the direct conflict with their own stick. Even that is not so bad in itself, but meddling with the sales of chromecast is pretty low.