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.
Isn’t this true for all protests? My understanding is that people who do them font agree to live in the constrains drawn by small number of raging lunatics with oppression tools. That’s why they organize. Into masses.