I was once called into the college VP's office and asked if I would consider teaching two courses at an extension campus for free. The classes were held in Baptist Church about 20 miles from my home. The College was paying a could of Cuban preachers, who resembled Mutt & Jeff, $4000 each to use the facility. They did not pay the Church, just the preachers. Their job was to stand around while the faculty taught the classes and act like they were in charge. My job was to teach English composition to Cubans who barely knew English. The college did not provide books, or even a place to buy the books, so I paid for the books and sold them to the students, some of whom never paid me. The salary mentioned to teach the course was $500. I had 30 students, and the college was getting $900 a head from them or perhaps the government for each one of them.
When I was asked to teach the course for free, I had already taught half the semester. So I told the VP, "If I work for free, that means that my work is of no value to you. But even if that is so, it is of value to me, so no, I won't work for free. "
Later, a colleague said that the rumor was that I was "uncooperative".
No one should be allowed to have unpaid interns.