If you don't ack enough Uber rides, they will fire you. The drivers are exactly in the McDonalds situation described above - for practical intents and purposes, they can't leave the shop, but they are also not being paid to stay there.
There is only one reason for this policy - discouraging moonlighting.
How does Uber forbid moonlighting?
I know tons of drivers on both Lyft and Uber.
Uber is not paying people to be available and does not require you to work a certain schedule. They're paying you only for rides you pick up.
There are arguments for classifying drivers as employees, but time isn't one of them.