@urlyman
A better answer, from Abigail Thorn, years ago:
Because when commentators describe a group as violent or peaceful, they are usually not describing that group's activities but rather the social response to them that the commentator desires. If someone called Insulate Britain "violent", they were saying that they wanted society to use violence (in the form of the police and prisons) against Insulate Britain, not that they thought Insulate Britain were themselves doing violence.
A neat proof of this is by asking such a commentator about whether they consider the military or police to be violent. Those groups are unquestionably violent, that's their job, but people often don't like the idea of violence against the police, and so calling them "violent" is often difficult.