These aren't left wingers in the liberal left winger sense. Practically
everyone in San Francisco is a liberal left winger. These are
militant left wingers (the photos of the event seem to suggest this, based on my experiences in the neighborhood). I would not be surprised if there were more than a few violent folks in the crowd.
Keep in mind what happened the last time there was an anti-gentrification protest in the Mission. Ostensibly it's a peaceful gathering to march against gentrification. It turned into smashing windows of local independent businesses and massive vandalism.
Every leftist protest or demonstration in San Francisco seems to have a bad habit of turning violent. This isn't to say that everyone there is violent, or that their cause isn't worthwhile, but rather that if you see these marches or protests, the Black Bloc is always close at hand, so I'd watch right the fuck out.
> "I would be very surprised if the people involved in an open air drug den had much to do with the people attending and organizing this protest."
There's not much of an intersection. The whole open-air drug den thing isn't to associate the protesters with the crackheads of the neighborhood, but rather to say that this isn't like any street corner in San Francisco. This is a street corner that the police effectively have no control over. If you get jumped by a Black Bloc asshole with a bone to pick with Googlers, you are fending for yourself. Though, reading the article, it looks like the cops were on hand for this one, which is a nice change for the corner of 16th and Mission.
All in all, don't be so cavalier with your own safety. While San Francisco isn't ruled by the Khmer Rouge, the Mission is not a safe neighborhood. Don't let the bistros and boutique coffee shops fool you. If you mind your own business you'll be fine. If you go looking for trouble, the Mission has no problem delivering trouble to your face, fast.