My immediate reaction was “Given the crowding in the sector, and that they presumably earn non-pecuniary returns from the enjoyment of teaching, shouldn’t we be taxing them at a higher rate?”
He is referring to the low salaries for adjunct professors. A college that pays different salaries to full-time faculty and adjuncts is engaging in price discrimination (actually, wage discrimination). Just as a price discriminator tries to charge based on willingness to pay, the wage discriminator tries to pay according to willingness to work. Like it or not, low pay for adjuncts is an efficient outcome.
With price discrimination, a seller charges customers a different fee for the same product or service.
And adjuncts are often better teachers than those with tenure, who were hired and retained for their research.
Maybe, but price discrimination implies market power. Do we really believe that universities exercise monopsony power? Competition for star researches belies market power generally, so the onus is on showing that the adjunct market differs substantially. Perhaps market power is plausible for some very narrow disciplines that yield few outside options, individuals tied to a location, or those for whom teaching provides very large non-pecuniary returns.