How about the tolerance paradox? Tolerant society that tolerates intolerance will cease to be tolerant.

More critically, it's to protect that speech from the government. Because you don't really need protection from anyone else, as no one else has a monopoly on violence.

An online mob can turn into violence. Or, the least look to silence opinion, either directly or by soliciting favor through boycotts, etc.

The violence of an online mob is not protected by the government. So who cares? Arrest the mobsters and move on with life.

A boycott is not even violence. So, again, who cares? We're not even in physical danger in the case of a boycott. In fact, the boycott is actually free speech. (Or, more precisely, free expression).

But consider, if the government comes after you, who will you arrest? Who can you get to defend you? In this case, the Constitutional protections of our freedoms are essential.

The threat from the government is the danger that the Founders were insistent on mitigating. And they were right to do so. The government is the main entity that we need protection from vis-a-vis restriction of rights.