i am far too lazy to do any research, but as i recall, the sidewalks are kind of public, kind of private. so, for example, a business can declare no loitering or no smoking, but i don't think they're allowed to remove the sidewalk entirely.

as ayelle put it, someone has to do it, and someone has to pay for it. personally, i think the city is porbably saving taxpayers money and avoiding corruption by having homeowners do it themselves rather than hiring a company to do the city