This is an old recycled study and far from being scientific. They assume causal relationship (yup!) and it's based on surveys. It basically looks like the authors had their pet theory and then build the paper around that to make a perfect headline maker to fuel government agenda. The simple fact that people with mental health issues self-medicate wouldn't stroke prohibitionists. Cannabis is so popular that person with issues sooner or later try it and find it helps them. This study is essentially the same as finding people taking paracetamol for headache and saying paracetamol causes headache. The stronger dose people take, the stronger headache they have.

I think publishing such things should be illegal because it causes harm to public.

The original study: http://www.thelancet.com/pb/assets/raw/Lancet/pdfs/14TLP0454...