The concept has had various names but it is certainly not limited to Radio. In large part because so many post-secondary programs now (beginning in the 1990s) require students to spend all or part of one term/semester working, often for free, in their chosen field, before the school will graduate them. In other words, these school programs have flooded the market.
At the other end of things, broadcasters and employers in other industries where schools require students to work in their profession before they can graduate are now taking in a lot more students. Free labour, yes, but it also costs these broadcasters and employers a lot of time and effort training and sometimes babying these students without the pay-back they would get from an employee they are hiring. In other words, on average, Interns don't stay very long, even accounting for those who get a full-time job at the same place after they graduate.
For the record, here in Edmonton, most radio stations pay for more student hours than they get for free, when you take into account the large number of students employed (paid) as Operators and at Remotes while they are going to school.