There must be some reason why Uber doesn't already charge drivers for cancelling (must be insuring against some scam pulled by some pax). The thin vieled threat of being deactivated for having too large a cancellation percentage is obviously anything but a threat.
In another case, the system would have to work out a way to punish or prevent drivers from simply wasting time after a job is accepted, which then compels the customer to cancel instead (often they accept a job and then sit still). This can cost the customer a cancellation fee (before calling Uber) and it doesn't incur a black mark of any sorts against the driver. Of course, if your driver really was coming but might have hit traffic or some unfortunate misadventure on the way...
