You are absolutely correct Nick, it is cheaper to have a pre booked local taxi than one of the "airport" taxis both of which are legal and you can get it even cheaper from some other sources, however these are illegal and the authorities are taking steps to prevent them from operating.
Certain paperwork has to be provided as proof of legitimacy, and while it's fine for a mate to pick you up and drop you off, those "illegal" ones who make too many trips to the airport are being watched and face pretty heavy fines.