>In a society where no limit seems to be placed on the earnings of sports stars and others in their early twenties, why should doctors be expected to restrict themselves to less than their market can bear?

Because setting a price for maximum profitability means that whoever can't afford medical treatment dies

People not dying is probably more important than profit/free market/ensuring people can buy mercs or giant yachts or whatever, socially/politically/morally speaking. though if your opinion differs and you have a good argument for it, I'm happy to hear it.