My hunch is this:

The courts have held that, according to the doctrine of fair use, I may take a recording I own and make a copy in a different format for my own use. But I'm not permitted to mass produce it. Once the tunes are digitized and copied, how do keep a customer from making many multiple copies (presumably for resale) on your equipment? And if the tunes are stored for the use of anyone else other than the original owner, you've clearly got a legal problem. Can you devise safeguards to prevent people from doing that on your equipment?

--Charlie Baum
(disclaimer: the above is not really legal advice)