I wouldn't expect stats to work properly for side loaded KePubs where the source ePub was mangled in some way that isn't compatible with Kobo's assumptions about how a KePub is laid out. Unfortunately, that does seem to be a number of books even from major publishers.
If they really are hiding stats from side loaded KePubs because stats don't work on books that aren't supported and shouldn't be there anyway... it seems to me they're just trying to annoy people. If they wanted to actually deal with the "problem" of things not working properly with side loaded KePub files, why not prevent them from using the renderer? I don't like that explanation. Maybe there's more to it but as-is it sounds like an excuse.
Dear Kobo, please don't actually block us from accessing the KePub renderer. Pretty please
