SFO does not have any TSA checkpoints!
Remember, TSA does not conduct passenger security screening at SFO. This function has been outsourced to a private contractor. If you look closely at the TDC/TSO uniforms, you will see that they are similar to TSA uniforms, but different in some key ways. They say "TEAM SFO" on the back, for example, instead of TSA.
So, this procedure might be something cooked up by the third-party contractor managers, not any kind of directive from DHS.