That's not entirely accurate. A guideline for unpaid internships is that the interns should contribute nothing to your business; they get to learn and you get to train them so you might get a better shot at employing them later, or at least know if you don't want to hire them.
What's cool about our industry is that interns can contribute a lot, on or soon after day one.
Sure, but then they need to be paid. That's why I said it was a guideline for unpaid interns.
If you don't pay them something for the work they do, that's sometimes illegal and definitely immoral.
The guidelines are also different for nonprofit and for-profit companies (and if your interns are getting college credit vs. not), so it definitely pays to check. FWIW we pay our fulltime interns, but there are circumstances when it's ok not to.