How do you know that those people aren't paying?

OTOH, I don't know how you could effectively enforce that single occupant vehicles are paying.

When I worked for a tollway (not SF so maybe they're different), toll violations were enforced by mailing a ticket to the offender after the fact. There weren't any patrols out on the road looking for violators. Don't pay the fine (plus the toll), don't get to renew your license plates. We had agreements with some other states for enforcement against their vehicles in our state. The cameras rarely were unable to get a good enough view of the license plate for the CSRs to not be able to find out whose vehicle it was.

I didn’t miss the point. You had zero way to know if someone is in violation or simply paying to be there.