I am going to discuss the concept of "free speech", not government regulation, because I don't think anyone at Charlottesville was arrested for just marching.
The issue I have with the whole rational argument thing is that having to argue constantly and entertain every other opposing view requires me to expend time and energy to understanding viewpoints I disagree with. Why is the onus on me to understand others instead of them having to convince me?
This is a debating tactic: you put the other chap on defense, forcing them to respond, which puts them in a weaker position. White supremacists, racists, climate deniers, and the like use this tactic online all the time, they pose dumb questions and make challenges, citing obscure sources that take time to prove false, all the while, they get what they want: they put you on defense taking time to defend the consensus while they have to demonstrate nothing in the meantime. These arguments get nowhere, because the people who posed them never were genuine in the first place. The intent was always to spread FUD and convince the uninformed that there's enough doubt to at least entertain the idea, and by the time you post your multi-paragraph reply (like I am here), most of the crowds' eyes have glazed over and the trolls already accomplished what they set out for.
It gets to a point where these tactics are so tired that internet communities silence these voices wholesale, like here on HN, we don't rehash emacs vs vi or things like that over and over again and admins generally flag political threads. I know dang says they do it to keep the community from turning toxic, but the reason it gets toxic is because ideologues aren't really interested in an honest conversation, toxicity is their goal as I alluded to above. On a more personal level, you learn to not really entertain people when they bring up the Bell Curve or how "global cooling became global warming", and that isn't being close minded, that is not choosing not to feed the trolls.