From CBS Local San Francisco, transit police just doing their (unnecessary, in this case) job with unnecessary force:
Video has been obtained by KPIX 5 showing a BART police officer who repeatedly used a stun gun to subdue a passenger in front of other riders.
A woman who witnessed the incident, who did not want to go on camera, told KPIX 5 the man was harmless and that the officer used the stun gun for no reason....
While witnesses were heard on the video saying the man had done nothing wrong and not bothering anyone, the officer used the stun gun on the passenger.
As riders look on, the video shows the man being dragged to the aisle. “Don’t move or I’ll tase you again,” the officer was heard saying.
But that was not the end. Moments later, the officer said to the man, “Get on your stomach or I’ll tase you again.”
Minutes pass and more officers arrive. As the man was being held down and handcuffed, the video shows the same officer using the stun gun on the passenger for five seconds.
Tasers can be potentially fatal weapons, as officers should well know. And BART's own policy says "that a stun gun can only be used if the suspect poses an immediate threat of bodily harm to the officer or another person."