Uber and Lyft don’t let their drivers set rates, but the argument (and law) is that they should — since Uber won a judgement saying their drivers were contractors, not employees, their drivers, then, are legally entitled the right to charging their own rate. For a lot of intents and purposes Uber/Lyft wanted drivers to be classified as contractors so they wouldn’t be subject to laws re: minimum wage for employees, but it seems also don’t want to be subject to all laws re: contracting (price negotiation).

Barring some extreme measure of lobbying for and passing a law that carves out an entirely new employment classification for the gig economy, Uber/Lyft/etc have to comply with at least one of the existing classifications.

They aren't, and the furthermore is also true beyond a certain percentage. This is part of the reason why in a number of jurisdictions the courts have ruled that the drivers aren't independent contractors.