I wonder if they bother justifying moves like this to the consumer. Do they try to spin it as a positive somehow, or do they just do it quietly and hope folks don't bother them about it?
Transferring it to your PC was never meant to be a backup. It was so you could then transfer the book to your own kindle and was a relic from the early years of Kindle when wifi was not ubiquitous as it is today. (The same reason early kindles all had cellular built in.) The downloaded copy had DRM on it and could only be read on an Amazon device licensed for the book. However, it was trivial to remove that DRM because it was based on a very old version. A few years ago Amazon moved to a type of DRM that was not trivial to remove and they were closing remaining loopholes.