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.
And Uber is free to not offer you any more work
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.
This would skirt pretty much all employment law.
This is a difference of how people view the platforms, I dont believe that is either fair or correct.
If I sign up for say a Freelancing site that offers a wide range of services I have not "tacitly agreed to" offer every service the site offers, I have not "tacitly agreed to" do the same thing that everyone else on the site offers
That is what uber is a Freelance Driver site, where a customer can select a Driver to perform a service (take me from a to b), just like when I take a Freelance programming job I have (and uber drivers should also have) the ability to set the terms of who they accept as clients, and where they want to go
That is the very nature of Freelance work.
A Taxi Service however would be different, as it would be if Uber was a "Car Service" that employs the drivers directly, and to be clear there are Car Services out there with company employed drivers they are $$$ but they exist
Each Trip is a "Piece of Work", is a contract, is a Freelance Job.