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.
For all of the unequivocally civic commenters on this thread, ask yourself this: would you be defending someone driving through a protest if the protesters weren’t on bikes?