It's very easy to attribute actions to the police, since they very commonly wear easy-to-identify uniforms and it's illegal to impersonate them. The video above was of a uniformed police officer in the company of maybe a hundred others. It's the literal job of police to stop crimes, so if you see an officer commuting a crime and his colleagues see and don't intervene, you can assume some level of assent to the officer's criminal activity.
Even if you found a video of a looter/arsonist, how do you know they're a protester at all? If they are a protester, how do you establish what their faction is (BLM, anarchist anti-capitalist, Neo-nazi agitator, etc.)? How do you judge the attitudes of the other protesters to to their actions, given protesters aren't equipped to enforce the law and don't have the mandate to do so?