There are specific criteria that should be met for an internship to be offered without payment. Generally, an unpaid internship must be purely educational, not displace employees, employer does not derive profit from the intern, and there is no expectation of a job offer at the end.
At my employer, interns are effectively additional staff for the duration of their employment, so they are paid market wages. The main difference between their position and that of a junior employee is hourly wages vs salaried.
A quick Google suggests some difference in the US, in law if not practice. In particular that the internship experience is for the benefit of the intern and the company derives no immediate benefit from the intern's activities (and may even be impeded by them)
At my employer, interns are effectively additional staff for the duration of their employment, so they are paid market wages. The main difference between their position and that of a junior employee is hourly wages vs salaried.