[cperciva]

From the article:

While I'm playing, I'll routinely see a policeman ask a person sleeping on the ground to sit upright. I'm not sure if that's because of the civil sidewalks ordinance or to "prevent obstructions".

I don't know about San Francisco specifically, but in some cities police forces use this sort of instruction to identify people who need medical assistance: If you're able to respond to the police, you're probably fine, but if you just lay there then you're probably incapacitated by alcohol, drugs, or some other medical condition.

[bigdubs]

In NYC (and I think San Francisco) it's illegal to sleep on the sidewalks. NYPD enforces this pretty consistently, SFPD on the other hand I'm pretty sure they just do it to make sure the person isn't dead.