Kinda a no brainer that mind altering substances aren't for the developing mind....
But studies like that are not really a fair representation either, regardless of its effect on the developing mind (And I would not try to argue that there is none)... Marijuana users are not a fair cross section of society, so comparing the success or intelligence of marijuana users to those who do not use is not fair.
I bet a similar study could be done with top ramen noodels. Study a large group of children through their lives, I bet that you would find less mental development and intelligence in those who regularly eat cup o' soup or other canned foods... Does this mean Spaghetti O's hinder mental development?
The actual danger/problems aside, societies view of cannabis is what it is and from youth children are taught to "say no to drugs". It would make sense that those who are prone to listen to their teachers and parents, take an active role in their own development and focus making positive life decisions would be more likely to follow the message to stay away from drugs and alcohol... and that those who live carelessly and do what they feel like would be more likely to experiment with drugs and less likely to focus on studies.
Again, this is not to claim that there are no negatives to marijuana, just that this type of study really has some pretty big outside factors that make the numbers not necessarily valid.