strange! Since you can only apply DRM via selling it in the Apple store and turning on that option (or equiv for Kindle store), or if you have one of the uber expensive Adobe License Server subscriptions, my guess is that they’re doing something wrong. ;-D Could be as simple as opening a previous EPUB downloaded from the iBookstore and confusing it with yours?
I would ask them to email you the epub that’s giving them that message and see if you replicate the problem.
OR it could be they’re trying to preview it via BookProofer, which won’t work with EPUBs containing encrypted fonts. In that case it does throw an alert about not being able to deal with encrypted EPUBs.
AM