Seattle Mayor Jenny Durkan just said at press conference that the police don't turn on their bodycams in a protest, unless they are about to take some action like arrest. This is to protect the privacy of the protestors.
Meaning, the Seattle cops aren't going to have videos of people throwing stuff at cops, or looting, or setting cars on fire unless or until they go to arrest those people.
Sauce is clip from live YT, can't cite the time because it's live
https://www.youtube.com/watch?v=aH7yi0FmZEk