My understanding is that to implement the Cast SDK in an Android or iOS app, one must link a closed source binary. On Android, it's Google Play Services, while

My understanding is that to implement the Cast SDK in an Android or iOS app, one must link a closed source binary. On Android, it's Google Play Services, while on iOS it's a custom Cast binary that Google provides to developers to link in their apps. Due to this, almost all apps that support Cast are on iOS, Android, or Chrome because closed-source binaries are only distributed for those platforms. Amazon would have to make special arrangements to include the Cast SDK on Fire OS. I still don't think this excuses them from bringing a proper Amazon Prime Video app with Cast support to iOS and (Google's) Android.