Very few internships provide "no immediate advantage" to the company where the intern is working. For instance, if you were a publisher, and had an intern read unsolicited manuscripts and rate them, and then used those ratings to decide which manuscripts to read, that would be questionable. If, on the other hand, the intern were told to read the manuscripts that have already been accepted and the ones that have already been rejected, so as to learn what makes a good manuscript, that would be OK. That's not benefiting you--it is just providing training to the intern.

Essentially, from what I've read, it seems that what you need to give an unpaid intern is work that you don't actually need to have done, and have no use for.