Peer-to-peer car-sharing company Turo Inc. has fired back against a lawsuit filed by the San Francisco city attorney, claiming the charges San Francisco International Airport (SFO) seeks from Turo are an unlawful tax.

What’s more, the San Francisco-based startup claims the city attorney’s office is doing the bidding of multinational rental car companies—in particular, Enterprise Rent-a-Car and its parent Enterprise Holdings—whose fees provide a significant portion of SFO’s revenue.