I think in the case of professionals, supply to the market is regulated by professional standards boards which limit entry into the profession.

Due to this pre-existing restriction, I am not surprised the courts would hold that people within that sphere should be free to compete on price, as each doctor will (you would hope) have the same basic set of skills to an extent.

The above restriction is not in place with regards to e-book providers so I think this weakens the argument somewhat.