1A proponents need to understand that there's a grey area between outright incitement to violence and otherwise. Within that grey area lies conspiracy theories and hate speech which can motivate actual harmful and illegal activity downstream if these ideas become sufficiently widespread. It's plausible that the unmoderated hate speech on 8chan motivated numerous lone wolf shooters, given how many manifestos have been posted there.
While we may want this speech to be legal (I largely do), we should not want to voluntarily assist in the spreading of this speech above and beyond the legal requirements of 1A. Anti-1A sentiment is growing on the political left and that sentiment isn't totally invalid if we zoom into specific examples of speech motivating heinous outcomes. So nipping a real problem in the bud with private self-regulation seems to be the least slippery slopey outcome of them all.