First off, I question the validity of the study.
Second, even if the study is valid, in a world gone absolutely ing mad, how dare anyone try to define insanity against the supposed norm. If people are going psychotic after years of use, maybe, just maybe it's because they're able to see the world for what it is. Taking the scales from your eyes and looking at the around you would make anyone a little crazy, especially those predisposed to it. Doesn't mean people should just put their heads in the sand. I mean, we are living in a full on dystopia. And I hardly think it's fair to label the people who understand that and purposely cultivate that understanding, as psychotics.
Finally, to me, not smoking weed is ing crazy. Not hyperbolically. Literally. I think there is something clinically, mentally wrong with people who don't see the world through marijuana's lens.