I don't know what any contracts say, or may have said in the past, but the Uber website makes it clear that drivers are paid a pretty fixed rate based on time and distance. I'm sure there's some extra details around the high demand times and all that, but there's no mention of getting paid a fixed %.
Perhaps the contract changed at some point, and Uber decided it didn't need to honor the prior contracts (or was negligent in taking them into account)?
Edit: Not to imply the accusations are valid. I'm not trying to make a case that Uber must be guilty, as much as my wording could be read that way depending on how you approach it.
https://www.uber.com/info/how-much-do-drivers-with-uber-make...