One of the defining features of Uber is that the drivers set their own hours, too. That's how surge pricing is supposed to work--it's supposed to incentivize part-time drivers to get off their butts and drive during peak times.

Sometimes it backfires; Uber sent warning emails about surge pricing New Years Eve to drivers and to riders, and the cumulative effect of the warnings resulted in a glut of drivers and relatively little surge pricing in Seattle, at least according to a driver I talked to. In this case, leaving it as a contract arrangement even works for Uber because it lets them leverage market forces where a traditional employment model would make it impossible.