Amazon Reverses Course, Encryption Returning for Fire Devices 10 years ago
The most likely reason for encryption to be removed is simply that the two engineers on the Kindle Enterprise team who implemented the feature both left the company a year and a half ago. The remaining engineer on the device side portion of that team moved on to another project. The integration, security review, etc with the SoC vendor is fairly complex, and the expertise required to build out encryption was lost. The original integration took quite a while as well. Most of the speculation as to why Amazon removed it seems pretty out there.