At first blush, it sounds like an arbitrary restriction to keep ride-sharing chauffeuring in the company ecosystem, but the prohibition appears to be more of a legal issue. On the self-driving section of Tesla's site, a disclaimer explains that even turning on self-driving functionality depends on local regulatory approval. Ferrying family and friends is fine, but you may only earn money while their cars are in autonomous mode when operating under the company's forthcoming ride-sharing service, the Tesla Network.
That's coming some time next year, says the site, though no other details have been released since Tesla founder Elon Musk released his Master Plan Part 2 back in June. But as Ars Technica points out, the restrictions only apply for self-driving rides: Tesla EV owners can still use their cars to earn money so long as they're behind the wheel. How the company will tell the difference is unclear, but it's no secret that they collect drive data from customer cars.