A "quirk" here means "an accidental peculiarity, a strange detail in the rules, which has an unintended consequence." For example, "There is a law in Maine about overtime payment. The people who wrote it left out a comma. As a result of this quirk, truck drivers won $10,000,000 in a lawsuit."

This is a common thing. If a society becomes dishonest as a whole, no law can help it, because laws will just be circumvented in the same way that language can have many meanings. Here, the employer is getting around employee compensation laws. If you hire someone to do a job for more than 30 hours a week, you have to pay benefits like health or dental benefits because the job counts as a full time job. So, in this case, the employer gets around the spirit of the law while following the letter. They just employee a whole lot of employees and call them all "part-time" employees so they don't have to pay for health or dental benefits for anyone.

I think it must be a printing or writing error. I wonder if "quirk" should be replaced with: "They work"