FlightCar, and to a lesser extent the other off-airport rental companies (who still run shuttles from the rental car center), don't burden the airport with space in the multistory garage on the airport property. As such, they shouldn't be subject to taxation by the airport.
SFO's an interesting case because it exists completely beyond the boundaries of the city and county of San Francisco, with FlightCar's property even further afield in Burlingame. I fail to see how they could even have standing.