Missing the point
It has nothing to do with minimum wage, and everything to do with the relationship between Uber and it's drivers.
If an Uber driver can also work for Lyft, et al then great. The driver really is self-employed.
However, I suspect that somehow Uber will prevent Uber drivers from doing this - making the driver an employee and therefore protected (for now) by employment legislation.
Calling employees "self-employed" is a well-known (especially to HMRC) trick to allow companies to shirk their responsibilities. It's also defrauding the revenue .....