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.
This isn’t about flipping off a driver. This is flipping off a bad driver who doesn’t understand that what they’re doing is unsafe. They’re flipping off bad behaviour.