Could you please elaborate? I don't exactly follow. Nobody is forcing them to work for Uber. If what you mean is they have bills to pay, etc, it's not like Uber can just magically decide to pay $9/hr, they will likely need to cut most of the jobs at that point anyway.
The $3 markup only becomes apparent when a competitor is cheaper then the other players, in which case the competitor gets more orders and make more revenue. Now if there was a law, called minimum wage, which forces employers to pay enough to make a living, there would be no extreme price cutting. Still, about the same amount of people would be employed (maybe slightly less, but most people will still order the meal whether it is $20 or $23 assuming there is no cheaper alternative which fits there needs).
If we do not have those protections, we end up in a situation similar to the Industrial Revolution in Europe where people (and their children) work for 12+ hours a day and still starve to death. Those jobs would exist anyways because the demand does not decrease significantly if the wages rise (this is of course not true for every industry) but by having two competitors cutting down each other at the expense of the employee is bad for everybody if you consider the long term consequences.