Court decisions have been coming in favoring unpaid intern plaintiffs.
Articles in business media have explained in great detail how companies which do abide by the law are taking on inefficiencies. For example, the law requires unpaid interns to receive developmental training. As anyone who has overseen interns in that manner knows that involves plenty of work. Also, there's legal liability attached if the interns screw up.
And, of course, there is always the possibility of the experience blowing up in the media. Interns are unhappy. They give an interview to a liberal media outlet like The New York Times or a sensationalistic one like Gawker. They can also tell their story themselves in social media. In addition, if their parents have influence, there will be badmouthing at high levels. The company's brand could take multiple hits.
Yet, help-wanted sites such as Craigslist remain filled with requests for interns, mostly unpaid. They should know better. In Law.com, Joel Grossman explains all the reasons why this seeming expense-saver is bound to cost plenty in the future. Here you can read that.