Probably to discourage people from immediately 
throwing the cel up on Ebay (or another auction 
site) for a profit - if they do, then everyone 
knows exactly where it came from. And yes, this 
*has* happened before...

Another reason may be simply be because they dont 
have any other new cels to replace the images 
with. A store would probably rather have the old 
images up - to give you an idea of what they can 
get in - rather than having an empty page.