I'm guessing the reason for the restriction was the risk of people stealing it or razoring pages out - something that happens with some frequency with out of print art books.

It's even more of an issue with books which have images of obscure flora and fauna, at least with an art book if there's demand for a really good picture of a sculpture or painting you can pay someone to go take one, with pictures of rare mushrooms you'd have to go to huge lengths just to find them let alone get good pictures of intact specimens.