1) Uber is saying "I can get you from A -> B for $Y" and the rider agrees to pay $Y.
2) It tells the driver that it will pay them $Z to drive from A -> B and the driver agrees to it.
So what? Where is the fraud?
Consider a similar circumstance in Amazon:
1) A merchant is saying "I can sell you this widget for $X" and the consumer agrees to pay $X.
2) Amazon tells the merchant that it will pay them $Y to sell the widget to a customer. $Y here is $X minus Amazon's cut. The merchant agrees to take $Y in exchange for the product.
Is that fraud? Of course not!