If a customer has a problem with a particular driver, they contact Uber, they don't contact the driver directly. This is a key fact that makes a strong case for recognizing the entire fare as revenue.
The difference with Groupon is that the customers were still interacting with the end merchant, ie. whoever supplied the coupon, as opposed to Groupon as a whole.
One problem with recognizing the entire fare is that their margins will look a lot worse.