Drivers provide capital goods (a vehicle) and work (as opposed to hiring a bunch of employees which you'd have to pay even if there was no work) and they provide these at a moments notice at Ubers discretion.
If Uber allowed drivers to set their own rates that would initiate a 'race to the bottom' reducing the fares and thus Ubers percentage.
Because it is convenient to make these operators out to be independent entities for legal reasons does not rule out Ubers desire to treat them as unpaid employees for totally different reasons.
From Ubers persective some independence is good, just enough to not assume any responsibilities that go with operating a company like this but too much independence (such as control over the rates) is a bad thing.
The 'new middlemen' have to ride a very fine line here to protect their income stream.
A real free market is simply not in Ubers interest, they would like the prices to stay as high as possible, basically what the market will bear.