It makes zero sense. There's two reasons why it's 'wrong' for JWs.
-Illegal. This argument of course falls apart because it's now legal in two states and it's permitted for medical use in a few more.
-DEMONS. Yes, smoking weed can open your mind to demonic influence. However, it's apparently OK to use under a doctor supervision since it's OK to use opiates when prescribed by a doctor. This makes no sense whatsoever. When I go to light up does a demon stop to check if I have my medical MJ card? Probably not. He's not going to care. So if the demon thing reallllllly was a thing, weed and all narcotics should be forbidden to JWs.
It's just another case of old white men being old white men.