Even if that were true, it would mean the general population should be assured by a medical professional that they are not predisposed to mental illnesses before they use cannabis.
3 karma · joined April 16, 2014
MARIJUANA USERS HAVE ABNORMAL BRAIN STRUCTURE AND POOR MEMORY - http://www.northwestern.edu/newscenter/stories/2013/12/marij...
Persistent cannabis users show neuropsychological decline from childhood to midlife - http://www.pnas.org/content/early/2012/08/22/1206820109.abst...
From personal experience, I smoked recreationally from 18 to 24 and definitely felt serious, structural changes in my brain. I won't go into the details since it's anecdotal evidence, but if research like what I linked to and the OP was presented to me, I would have dropped it.
People need to be aware there are consequences to your mental development if you smoke marijuana casually. Obviously the government has blown them way out of proportion, but the problems still exist.