My guess is that Uber subtracts it from the amount that Uber would pay the drivers for the credit card transactions. They still support and encourage use of credit cards.

If a driver didn't deliver cash to Uber they would probably be deactivated. If they had an excuse for it they could try customer service.

In another comment femto113 points out that provided there's enough credit card fares drivers can keep all of the cash they receive, with Uber taking their cut via the credit card transactions. (However, it looks like Uber's cut is 20%, so it doesn't look like there's enough to make this balance out completely.)

I guess on longer term this is a non-problem, since uber provides the rides and controls the reputation system. So, if the driver doesn't pay the share, he/she gets kicked out of the system.