One thing occurred to me that might explain some of the artist/collector disagreement on this issue: Do some artists not realize that for many products that their sketches are the main selling point of the product?
I've seen several references from artists in the different venues that the sketches are "freebies", "bonuses", or "nice extras".
Maybe they don't know that their work is what is selling many of these products?
My sketch collection: http://sketchcollectors.com/gallery.php?user_id=727
Claim to fame: Coined the term BODA