According to the complaint, this was part of the contract:
52. The Uber Defendants and Plaintiff and the other Class Members had a
contractual agreement regarding the collection, receipt, and payment to drivers of the
fares paid by Users for the driver’s transportation services.
53. The Uber Defendants agreed that they would collect and pay to the
Plaintiff the fare by the User, minus a contractual service fee and booking fee.
54. As a result of the misrepresentations and omissions alleged herein,
including the Uber Defendants' failure to remit payment to the Plaintiff and other Class
members of the full amount of the fare (after deducting the contractual service fee
percentage and booking fee), there has been a violation or breach of the agreement
between Plaintiff and the Uber Defendants. Accordingly, Plaintiff and the other Class
members have been underpaid for their services and did not receive the benefit of
their bargain.
If the contract with drivers did, in fact, say that Uber would pay out the fare that the user paid minus a standard service and booking fee, then it seems like this is pretty solid, though IANAL. I have no idea if that's what the actual agreement is, though.
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.