Correlation does not prove causation; it could well be that pot smokers tend to more often engage in some other unhealthy activity that reduces circulation - or that some pre-existing condition predisposes one to both pot use and reduced circulation.
The problem very well could have a relationship to the sedentary lifestyle and poor dietary habits of marijuana users. However, I have already told you in a previous post that heavy marijuana users have already been known to have circulatory problems. This study of circulation in the brain only reconfirms this known risk. At some point correlation does begin to have a strong association with causation.