I think I remember back in New Leaf, there were rumors that dream towns with certain hacked stuff could mess up your game just by visiting them in a dream, even if you didn't hack yourself, just having that stuff, like, loaded in your game could cause problems. I don't know if that was true, but I heard about it, and people may be concerned about the same thing now. Which is why they might report islands with hacked stuff--they might be legitimately concerned that it could damage other people's save files or something. It might not just be jealousy or spite or anything like that.
I mean, probably there's still some of that kind of thing going on, too, but giving people the benefit of the doubt...