I'm a longtime NYer who recently moved to SF. for the past 10 years I've commuted by running each way, ~3-5 miles each way. plenty of experience with bikes and cars in urban areas.

Biking on the sidewalk is WAY more common in SF than in NYC, especially in manhattan. In SF, there seems to be a few main reasons:

1 - Biking on the street would clearly be unsafe. An example here is 3rd st between portrero and soma.

2 - Biking on the sidewalk on the block where you are stopping. These are people that usually bike on the street, but pull onto the sidewalk when they are leaving/entering a building. I understand this, but wish it wasn't so common.

3 - ignorance of, or just not caring about, biking rules and conventions.

It's easy to assume that #3 is always the case and get angry about it, but it's frequently #1 or #2 and I do think some empathy is called for.