This was a Critical Mass event, which is why the bicyclists are taking up all of the street as a way to reclaim the streets and protest the lack of safety for riders under usual conditions. It’s not legal, but protests are never useful if they’re fully legal now, are they.
Honestly. Assuming you make sure people aren’t actually in the cars before lightning them on fire, that would be a much safer, and much more efficient way to protest. No one is going to park their car in bike lanes if there’s a high chance they’ll come back to a charred mess.