Fire OS is a customized version of Android that Amazon uses to power its Fire tablets and smartphone. However, the latest Fire OS 5 release has deprecated encryption support, leaving owners of Fire hardware with a tough choice; abandon encryption and put their data at risk, or run an outdated operating system.
Fire OS 5 is the first release to use the Android 5.0 "Lollipop" codebase, and as such it is possible that this removal is down to a technical issue (such as battery life or performance). Last year Google reported that it would allow hardware makers to decide whether or not to enable encryption-by-default because of performance issues on older devices.
As of the time of publishing, Amazon has not responded to requests for comment.
Update: Amazon has responded to comment with the following:
In the fall when we released Fire OS 5, we removed some enterprise features that we found customers weren't using. All Fire tablets' communication with Amazon's cloud meet our high standards for privacy and security including appropriate use of encryption.