Yeah, it's silly and odd and likely done to push customers towards formats that they have greater control over.
Those epubs that aren't really epubs, randomly disallowing azw3 files (that they support officially!!!) from being downloaded directly from the kindle's built in browser and other restrictive behaviour are part of this. That's why I'm eventually looking to enable epubs on kindle once the people at mobileread find a way to do it. Apparently calibre can be set up to send files too via email so that's another option.
If they tied a bookwyrm comments section to an ISBN number for example then anybody/site could easily have it embedded to make it a universal tool rather than specifically connected to a piracy site.