I have to say that I got scammed by Uber drivers many times, when the region had dynamic pricing (not a fixed price at the moment of booking the ride). They took detours, ignored directions when I told them how to reach the destination in the most direct way (trying to make the best out of it). Always reported it, and got back the money most of the time.
So I think they have to deal with A LOT of fraudulent riders.
That's one of the many problems they've assumed the risk for (along with the reward of profit for solving). There are any number of creative ways to tackle it. Exploiting and abusing labor is not one of the tools they get to have in their belt to do that.
In other cities they have to use the official taxi price, that is calculated by distance and time. So this is different for every city and also changes over time. Fixed prices are much better for the customer, because you know how much you pay upfront and the driver tries to get you there on the shortest and quickest route.
So I think they have to deal with A LOT of fraudulent riders.