If anything, I'd say it technically means "it was legal when they were rarer, but now that they're becoming more common, it's illegal" as it's the growth of unp

If anything, I'd say it technically means "it was legal when they were rarer, but now that they're becoming more common, it's illegal" as it's the growth of unpaid internships, not of unpaid interns, that may be illegal. They were trying to fit the fact that unpaid internships may be illegal and the fact that they've been growing all into one title, and it didn't work so well. But splitting hairs a) is kind of annoying, and b) doesn't add anything to the conversation about the content of the piece.