Well, they claimed they removed non-Amazon devices because they didn't support Prime Video. However, for stuff like Chromecast, the burden lies on the app developer (which would be Amazon) to implement Google's Cast SDKs to provide support.

First read: http://www.androidauthority.com/amazon-new-app-forced-google... to understand that Google won't allow competing app stores. (https://play.google.com/about/developer-distribution-agreeme... for more details section 4.5)

As per this document: https://developers.google.com/cast/docs/android_sender You need Google Play Services (see https://en.wikipedia.org/wiki/Google_Play_Services#Concerns) to write an Android app with the Cast API.

Because of this, Amazon cannot distribute Google Play Services on their own Android devices and therefor cannot use their own Android devices to cast to a Chromecast. They could enable Prime Video on other Android devices but not their own. They feel this will cause confusion and therefor have decided not to write a Prime Video app for Chromecast. Saying "oh, Amazon just doesn't want to write a Prime Video app for Chromecast", while true, doesn't truly explain the complexities of the situation.

The business decision to remove Chromecasts from the network steps from the above desire to prevent confusion. I personally don't agree with it but I understand it.