Because what is insulting enough to warrant removal is extremely subjective. Pretty much everyone agrees there's a line somewhere, where people draw that line is very different. I would never even consider trying to stop people from making bad edgy jokes, but I will tell them the joke isn't funny and they just seem like a bigot if that's what I think.

I also wouldn't consider 4chan a public place. Someone holding up a sign in a train station that says "gays are an abomination" I would agree should be removed as they are actively making a public space hostile to someone. But then I'd also put different standards on a non-spontaneous demonstration, as those have to be cleared beforehand and won't happen on a super frequent basis, and there needs to be some way to voice any opinion that doesn't outright call for violence.