Taxi drivers significantly under report earnings for tax purposes. They always claim close to minimum wage. You'll find the same in every cash business.
Take it for what it's worth (sample size == 1), but there are a lot of people who would love to be driving a cab in SF. Cab drivers who rent the cabs pay $300 for an 8-hour shift on weekends (Friday evening - Saturday morning), and still manage to make enough money to make it worth their while.