Turns out, though, those carpooling features are breaking the law, according to regulators in California.
CNET's Dara Kerr reports that the California Public Utilities Commission has sent warning letters to all three companies which say that it's illegal to charge individual fares to people traveling in one vehicle.
The letter to Uber, which CNET linked to, says that Uber may either petition for a modification to the current law or try to modify its existing permit to include the different business model that carpooling creates.
Business Insider reached out to Lyft, Uber, and Sidecar for comment. A Sidecar spokesperson told CNET that it is reviewing its options about how to proceed with the carpool feature.