1. How do you know that is actually done by the protestors, or some unaffiliated hooligans?

2. How do you know that the news coverage of the event will accurately make a distinction between the two? My experience with this is that the distinction that is made is always conveniently aligned with the political leanings of the news source.

Postulate: Maybe the 'peaceful' versus 'non-peaceful' demonstrations isn't actually a relevant dimension - and is a distraction, used to condemn some protests (that you politically oppose) as illegitimate, and to endorse others, as legitimate?

Every demonstration is going to have a few vandals and hooligans, either planted, embedded, or just standing on the sidelines. I postulate that it doesn't alter the legitimacy of their grievances.