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.
It’s not functionally identical, the function of the right lane is to move traffic in the opposite direction to the left. It would be absolute carnage if every other vehicle decided to disregard the rules of the road and use any lane they want because it would make it theirs.
Of course the traffic in their own lane has to stop when others are breaking the rules of the road as you don’t want to hurt anyone, but it’s still dangerous and stupid to ride a bike into oncoming traffic.