DOC says ......... Jws have some half-cocked idea regarding drug use (and even tobacco, I think) that links it to "spiritualism"
Ironically alcohol for centuries was called spirits in many cultures.
Based most likely on the notion that when one drinks this product one immerses themselves into the spirit world.
I can see some problems with marijuana and the JWS leaders for it might be taken in similarity to smoking tobacco for pleasure, which has always been legal.