From psychiatric times:
Many studies have explored the link between cannabis and psychosis (Table). In a systematic review, Moore and colleagues10 surveyed the literature on this topic. They looked at population-based longitudinal studies as well as nested case-control studies that assessed the impact of cannabis use on the later development of psychosis. The “psychosis” outcomes required the diagnosis of a primary psychotic disorder or affective psychosis, or the occurrence of delusions, hallucinations, or thought disorder during the study period. Results from 7 cohort studies showed a 40% increased risk of psychosis in cannabis users compared with nonusers. The data also revealed a dose-response effect—the risk of psychotic symptoms was increased approximately 50% to 200% in those who used cannabis frequently compared with nonusers. - See more at: http://www.psychiatrictimes.com/schizophrenia/cannabis-psychosis-link/page/0/2#sthash.UXUqJI4W.dpuf
The study only proves the known correlation that mentally ill are more likely to abuse substances.
The same exact correlation can be seen for tobacco use and mental illness. It only proves the mentally ill are more likely to use drugs and not care about their health. It does not prove the substance causes it.
There has not been a SINGLE before-after study or a SINGLE twin study showing ANY direct permanent changes from using pot in an individual. Neither in humans or animals.
Yet we allow people to consume alcohol without limit which is 100% guaranteed to rot EVERY cell in the body in EVERY individual
No, causation is a leap the researchers did NOT make. From your link:
"Whether cannabis can trigger a primary psychotic disorder that would not have otherwise occurred is unclear. However, in most individuals who use cannabis, psychosis does not develop"