Ummm.... most folks who have gotten tired of seeing their artwork reproduced by others.
You are really lucky to have cultivated such a trustworthy and loyal clientele that you would offer such detailed, labor intensive preliminary artwork without even so much as a copyright infringement clause being printed on it.
There are two things I have no hesitation shoving in a client's face BEFORE I or anyone else here spends more than even an hour's worth of design time on, a price range and material type or process. I'm an artist also , so if I want to fulfill my creative urges with something, I'll do a painting, not give away detailed, production ready sketches to anyone who calls.
Perfectly put.