Not mentioned in the article is the effect our (U.S.) health care system has in encouraging 'under-employed' entry-level workers.

Since the employer doesn't need to offer benefits (the most expensive of which is health insurance) to part-time workers, it's very common to give them 29 or 27 hours per week.

Contrast this with the salaried worker, who is measured by work product, and may work 60 hours per week to justify the salary.