Predisposition and causation

Study finds genetic links between schizophrenia and cannabis use | Reuters

Some pro-legalisation people will probably think this study helps throw doubt on cannabis as a cause of schizophrenia:

 The results chime with previous studies linking schizophrenia and cannabis, but suggest the association may be due to common genes and might not be a causal relationship where
cannabis use leads to increased schizophrenia risk.Cannabis is the most widely used illicit drug in the world, and its use ishigher among people with schizophrenia than in the general population.

"We know that cannabis increases the risk of schizophrenia. Our study certainly does not rule this out, but it suggests that there is likely to be an association in the other direction as well – that a pre-disposition to schizophrenia also increases your likelihood of cannabis use," said Robert Power, who led the study at the Institute of Psychiatry at King's College London.

But isn't the point this:  even if you are genetically predisposed to both use cannabis and get schizophrenia, does actually using cannabis help bring on the schizophrenia you're genetically predisposed to get?  

And as for the finding itself, I think it's hardly surprising.