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.
That’s literally what happens during every parade. They don’t close down half a street and let regular traffic proceed down half a parade route. That’s dangerous. You close the entire street and send the parade down both sides of the street.
Have you really never been to a parade before?
Right. You close the entire street, with barricades/detours and such. I don’t know what parade you’re going to but the ones I’ve been to don’t parade down both sides of the street while giving cars the option to drive down them.
That’s common in the US, but elsewhere theres often parades and demonstrations that se the entire road without blocking them off. Drivers can either wait or backout or be stuck there while everyone goes around them.