True, it doesn't have to tell you when you work. But it does have to pay you a minimum wage. If a staffing company said: "work whenever and we'll pay you a minimum wage", then drivers would probably routinely drive at times they found pleasant, not profitable times for the staffing company. In this case the staffing company would start up restrict the hours a driver could drive.

A staffing company wants to reduce the amount of unprofitable hours driven. To do that they have to restrict working hours. It's not guaranteed but a highly likely outcome of the set up.