I totally interpret this as a move to prevent any sideloading and ensure data collection and ad injection are being done on their part. Can't easily prevent those on something that's not a fully baked Android.
Useless to the you, but due amazon these are loss making hardware that exists to sell amazon prime ppv so they would make more money if you and me didn't buy...
I agree about ad-injection and data collection, but if they wanted to prevent side-loading they would have done it by now, at least via the main route, which is the Downloader app they seem to have no problem hosting in their app store
Amazon doesn't have much to lose from allowing you to install x piracy app, in fact it's a selling point for a lot of people. yes a few people might not subscribe to prime, but I would be surprised if that outweighs the ad revenue and device sales
I have a hunch if they remained on android core and specifically prevented sideloading it would spark more backlash than simply switching to another platform. This way they gain complete control of the ecosystem with less backlash. Less tech savvy people will buy it, and more tech literate people will skip it altogether as they couldn't profit off them anyways.