Your statement makes several assumptions - none of which should be made, especially with Uber.
1) That the driver is from SF. In fact, very few of the drivers are actually living in SF because you can't afford to live in SF on a Uber wage.
2) That the driver is doing going back to San Francisco without a fare. Yes, rush hours tend to see traffic all one way, but rush hours also prevent drivers from doing very many trips so the typical outcome is rides on the side they are on. Drivers don't unilaterally go the opposite of traffic - they follow the rides they are assigned. If that is back to SF, fine but it could just as easily be driving to Sacramento, to the inland/Pleasanton, to San Jose, etc etc.
3) That Uber is doing this for the driver - as in the driver gets the charge. Never make this assumption; Uber has been caught numerous times giving drivers less than what passengers paid minus Uber's stated charges.