[sveiss]

SFO’s involvement is that they own or lease the roads and parking lots where the pickups happen, and so have the right to set conditions on their use.

Parking enforcement isn’t automatic at parking lots without barriers either, but that doesn’t mean paying a parking fee to the operator is a “voluntary payment” just because you might not get a ticket if you do skip payment.

As for how they could enforce it, it’s pretty easy to walk around outside the airport and spot the cars with Uber/Lyft decals, or multiple phones, or use ANPR to identify frequent visitors, or just ask passengers as they get into cars. Multiple options for enforcement.

The enforcement might end up targeting the drivers rather than Uber itself, but it would have the same effect.