Because otherwise you run the risk of being bigoted about bigots. Ironic. Keep them in the open, learn what they think, be prepared to shut them down at every available opportunity - show them the error of their beliefs again and again and again. History suggests that banning things doesn’t make the problem go away. It just drives it underground, out of sight, where they can mutate and coalesce without anybody from the outside knowing or realising… then we all be fucked.