>but pyramid schemes are illegal, and drivers who peddle scummy garbage to passengers are liable to get bad reviews, so there is no reason to expect that a lack of this would have to be because Uber prohibits it.

Multi-level marketing is still common, and often the payoff might be worth your rating getting tanked.

If nothing else, it seems like stories of this happening would be common. Even just "if you listen to my fifteen minute thing on timeshares, I'll pay for your ride."