Most bartenders make minimum wage and that alone isn't enough for how expensive it is to live in SF, so they count on those tips. Plus they have to deal with drunk dipshits all the time, and they just stand there smiling while people scream their stupid boring stories in their ear. They get hit on all the time by bragging egotistical douche bags talking about their start up company and whiskey collection. They have to memorize hundreds of drink combinations, while dozens of people yell at them all night to serve them next. They have to listen to all sorts of horrible music that they never listen to at home, and they don't get off til after 3 in the morning. It's not an easy job, so show some respect like you do to servers in a restaurant.
Anyhow, here is a youtube video I found on tipping that seems pretty accurate: