it's not theft. The driver contract says you get paid back for the time and mileage that you incur, NOT "whatever the customer gets paid". Besides, if the trip goes over the amount that the customer pays (beacause, say, for traffic, or an unmarked road closure) then uber pays out the full amount to the driver for the actual time/mileage incurred.

let's say that Uber posted really unreasonably fast times for the given route instead. Would you then argue that uber could systematically pay the drivers a lower rate, because that's the fare they pitched to the passenger?