Why would that matter? I would totally "fire" (i.e. terminate the contract) a contractor that didn't work when I needed him/her to (i.e. that didn't get the job done). Why would Uber need to be any different?
Incidentally, if it were a 2-party scenario where you required a specific contractor to be available on demand, you would probably be their legal employer. A mandated, regular schedule is indeed one the main tests when distinguishing between an employee and a contractor. I have worked as a software contractor in the US for over almost 15 years and the employers, especially the larger ones with proper HR departments, have been careful to define my work in terms of skills and deliverables, not hours worked.
If I don't weed out people to make the connections as efficient as possible, I'm not doing very well at my job of handling the logistics.