Tolerating groups that want to create a less tolerant constitution poses a special (and difficult to answer) problem for freedom of speech (see:
the paradox of tolerance).
Imagine for a second that, instead of the Alt-Right, the protesters were from the Muslim Alt-Right (aka. Al-qaeda, ISIS). Imagine if they were demanding to install a radical vision of Sharia law that made burning the Koran a mortal crime; forced women to fully cover their bodies; and dissolved other essential freedoms. Few people, least of all conservatives, would stick out their necks to defend ISIS' right to free speech (however rightly or wrongly) out of fear of strengthening, or out of disgust for, Islamic fundamentalism.
Now, imagine if ISIS had a credible shot at gaining influence by gathering a sympathetic audience from certain groups in society. We don't have to imagine because that exact scenario played out in Weimar Germany, which had fewer laws against Fascist speech than modern Germany. Fascists in Germany were able to rise to power entirely because their hateful viewpoints were tolerated.
If these guys were from the Westboro Baptist Church, which is tiny and universally hated, they would be a cute source of amusement and derision more than anything. But they're not Westboro Baptists; they appear to have grassroots support.