In the UK at least that's one of the major differences between employees/workers/self employed. If I'm an employee I'd have to take the work offered, but if you're self employeed you should be free to decline work. It's a spectrum trading rights of the employee for freedoms.
If Uber don't want to take on the role of employer, they aren't allowed as much control over how tasks are completed or what tasks are accepted.
That is an overly simplistic view. It's worth reading the tribunal conclusions for the case in the UK that concluded the drivers are workers.
Edit- a quick example of why it's not so simple:
If this is the case, all employers could say their employees are just contractors as they are free to have all the contractor freedoms however if the contractor uses them then the company is free to stop offering them work.
If Uber don't want to take on the role of employer, they aren't allowed as much control over how tasks are completed or what tasks are accepted.