You make an interesting point. Maybe facilitating the usage of sideloaded books is not among Amazon’s priorities. Yet I don’t know how much of that comes from malice rather than simply negligence or lack of interest.
It’s directly against their priority of influencing you to only purchase ebooks through their monopoly. Whether anti-competitive, anti-user practices are malicious or just a consequence of capitalism run wild, I don’t think there’s much of a difference