Therefore it’s tainted by the implication that cannabis is an illegal drug and the people in this study spent some portion of their life associated with regularly performing a illegal act... This is basically worthless from the foundation up since they can’t possibly account for all the ways this might have influenced a persons upbringing, from the socially acceptable nature of taking risks with the law, to associating with drug dealers who may (or may not) try to push harder more addictive drugs on their customers.
It’s going to be a while until we get similar studies from the parts of America where it was made legal for recreational use, but until then, we have to remember that everything has been tainted with the implications of the subject participants in any study being involved in socially “unacceptable” illegal behaviour which is connected to a huge tangled web of second order effects.