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.
Use your brain for two seconds. Don’t you think they already tried what you’re suggesting? If you were planning a bike protest, wouldn’t you only go down one side of the lane? Well, it turns out, the people running this already tried that years ago, and they found that blocking both lanes is the only way to do this safely. Otherwise asshole drivers will try to “just slip past” by doing things like taking left turns. Parades take up all lanes on a road for a damn good reason.
Your idea was already tried and found not to work.
Parades close down the street with barricades/detours and such so cars don’t have the option to dangerously drive at them.
By law they don’t have that option either.