Dear dstang,
Either from your query or from your profile, it is not clear where you are based. Hence, it is difficult to say what the law of the land is. However, Madhu is an expert in law and has stated the position. Therefore, I will raise a few questions before providing a weblink about the law in the USA that can be used as a guide only.
Where are you based, and how large is the firm?
What is the objective of the internship: to give an opportunity for the student to study the organization and its practices, or to help the student see how theory learned in college applies in practice, or is it to assess the student for a job offer? For example, as a student at IISc, Bangalore, we had to undergo industrial training every summer. We were only expected to observe and not contribute. Most students undergoing a sandwich course get paid internships in their 3rd year of a 4-year course, and they are expected to actively contribute.
Dinesh Divekar has given you a sound reason why one should pay for interns: to attract better quality students. Willy Franzen in the USA has written a good article on "Are unpaid internships Illegal?" at [Are Unpaid Internships Illegal? | One Day, One Internship](http://www.onedayoneinternship.com/blog/are-unpaid-internships-illegal/).
It lists six criteria that should be met if an intern is not going to be paid. I suggest that you kindly read and see if it is of any help.
Also, please see [Buena Vista University - Paid vs. Unpaid](http://bvu.edu) (link updated to site home). Both found at [Google](http://www.google.co.uk/#sclient=psy&hl=en&source=hp&q=internship+paid+ver sus+unpaid+legal+issues&aq=f&aqi=&aql=&oq=&pbx=1&f p=d56a1d449790013d).
Have a nice day.
Simhan A retired academic in the UK