Many SF landlords have voluntarily removing units unit from the market by simply not rerenting after they become vacant because of all the hassles of rent control. This is one of several way that rent control reduces the number of units available for occupancy, known as "supply".

Understand? I'd not, I suggest reading any basic economics text.

Rent control in standard economic theory

http://homepages.wmich.edu/~u5nwaogu/Overhead/Chapter%206%20...

Rent control in practice in SF

http://www.nytimes.com/2013/06/07/opinion/king-of-my-castle-...

http://mjperry.blogspot.com/2011/05/because-of-rent-control-...