I get very irate by people parking on the sidewalks (and I have many photos of such transgressions, which I tag with "vanscaping"). A majority of the time, they are blocking off the entire sidewalk (or enough that a stroller or wheelchair couldn't get through), when if they had parked entirely on the road there would have still been plenty of space for cars to get past. A case in point is this van on Gilmour at O'Connor.
But another reason is that sidewalks are only reinforced where there is a curb depression--i.e. where it is expected that cars will drive over them (with some exceptions like sidewalks over a bridge). Otherwise, for the post part, they are straight-up poured concrete.
Back in the winter, Eric posted at West Side Action about cracked sidewalks on Preston, likely from trucks having driven and/or parked on the sidewalk. In the photo above, a contractor's van is parked on the sidewalk, and the van's right tires correspond rather conveniently with the cracks down the middle of the sidewalk.
Individually, people who park on the sidewalk think that they are doing no harm. But collectively, as many people vehicles are parked on the sidewalk over time, the sidewalk breaks down and needs expensive and time-consuming repair. In the meantime, they are harder to keep clear of snow, and water that seeps into the cracks worsens the problem during the annual freeze-thaw cycles.
[Look for more one-photo posts under the label Singles]