It's not fraud to charge different customers different amounts of money for the same service. Fraud requires misrepresentation; failing to volunteer information like "you could get this service cheaper if you paid a different way" wouldn't generally rise to that level.

It may be a breach of contract between Uber and its payment processor, though. Often, credit card merchant agreements prohibit businesses for charging more for credit card payments than for equivalent alternatives. Maybe Uber is just flagrantly ignoring those terms, or maybe it has negotiated its way out of them.