Looking at this page, it seems like they can't set their fares, but only a "fare multiplier", so effectively, Uber still decides.
Are there any restrictions on what the multiplier can be? If not then they can effectively set the fare. The "standard" fare is $X per mile and I want $Y per mile. I just set the multiplier to Y/X
They say the "fare multiplier can range from 0.5x to 5.0x in increments of 0.1". Not fully free, but being able to set over 5x the regular price seems mostly academic.
That's interesting, because there's much, much more granularity above 1.0 than below it if you enforce increments of 0.1.