Should interns be paid regular rate, not be paid, be paid half the rate, or other? See detail?

I mean interns, as in nurses’ assistants for example or producers of shows, and others who never get paid. Some people say they shouldn’t because they are learning just like they are learning in a classroom and more. They don’t ask to be paid for learning in a classroom, so they shouldn’t ask to be paid in another location. And they are not replacing the regular employee/s. And some say they should get paid since “they are doing the work.”, Should they get paid nothing , or ¼ reg. rate, ½ or or other?