I might be confused, but I thought the private security people at SFO (and at other airports with similar arrangements) were the first people you see, who look at your boarding pass first (not necessarily check your ID) and direct you to which line you should be in.
They might send you back to the ticket counter if your carryon looks too big, or if the items you are carrying are too numerous. They might also ask to see proof if you venture into the special status lines reserved for first class or sky priority passengers. They might also check your BP in order to prevent or permit you to go through the PreCheck line.
And that is where the private security function appears to end. Once near the screening machines, the uniforms change to TSA, and it is TSA personnel who are actually putting your ID under the special light. And it is TSA who is staring at the xrays of your carryon on the belt as well as the carrion behind the belt you removed.
I've observed the security uniform differences between the sheep herders at the beginning of the line, versus the screeners at the machines, even in SFO. But then again, things change, and it has been a few months since I've flown from there. Yet I'm excited to pay my first visit to the new SFO SC next month!