There’s quite definitely a technical problem with banning public street protest (i.e. mobs). Even a riotous mob is a vague concept, reliant upon discretionary police judgment on occasions. But is the criminalization of public protest also a problem of principal?
Strangely, most libertarians seem to think the right to free-association extends automatically to mob formation. This presupposes that a mob is not inherently an act of aggression, existing solely to intimidate, and in fact — strictly speaking — an instance of terrorism. It is obvious why the Left should like the mob. It self-identifies as the articulate representative of the mob. Far more obscure is why anyone from a liberal tradition, let alone further to the right, should concur in this appreciation.
Free expression hardly requires physical aggregation in public places, with near-inevitable expression of a potential for violence. It is not difficult to see that the basic historical role of the mob has been to advance demands, backed by implicit threat. Between a mob, a riotous mob, and a revolutionary mob, there are differences of degree rather than of kind. Even the strongest supporter of the principle of ‘voice’ should see zero additional value in its physical concentration. Resonance and group emotion undermine a statement, rather than reinforcing it, unless the ‘statement’ is collectively directed anger (which is to say once again, inherently Leftist).
Mobs are no doubt almost impossible to effectively criminalize. That does not at all mean one is compelled to like them, or acknowledge their legitimacy. Their existence is an intrinsic threat to both liberty and authority.
Perhaps laws against public indecency could be applied to politics in the street? In any case, it is past time for everyone to the right of the Left to lucidly despise it.