This is very frustrating. But it does serve as yet another reminder of the dangers inherent to closed ecosystems. I think it is great that I can cast my Amazon Prime Video from the app to my Chromecast, but I have been around long enough to remember that was not always the case (and I also will not be surprised if (when) I wake up one day and that functionality is no longer available). And don't even get me started on the joys of trying to use a Chromecast from an Android device without the Google Play Servies rootkit....
More and more I find myself investing in open systems (playing my own movies off of my Plex server...).