The issue with Taxis, in particular, is that they need to be regulated in some way. When hiring a taxi consists of standing on the curb with arm extended, while someone ducks out of traffic to pick you up, there is very little negotiation that can be done. It makes sense that, within a city, the economics of a taxi transaction be defined in advance.

That said, I fully believe Uber should be allow to do its thing. A transaction initiated by Smartphone App allows the passenger sufficient ability to consent to whatever deviation from "standard" terms the driver may provide.