There are regulations that prevent a doctor from discounting. Discounting one patient, you see, means that he's "overcharging" the insurance companies . The HMOs and others would then have the means to cut his fees to the discounted rate he gave you, which would surely drive him out of business when they then take their cut out of the lower fee! Medicare is particularly sticky on this...you can't even deal in the kind of transaction at all with someone over 65, or the doc risks being cut out of Medicare altogether.
They call it "fraud" and it is certainly absurd and unfair. But you're talking rich greedy doctors..