The title is incorrect. The why is unanswered and still unknown. The article simply states that there appears to be a correlation.
The downside to this body type is that the brain is always very active; it always has lots of calories to draw from. Makes for great programmers and problem solvers, but also leads to poor attention span and insomnia.
So my theory is that marijuana is a dampener on the brain for these people. It doesn't improve insulin control, it's just that people who like to use marijuana tend to have fantastic insulin control. The logic being good insulin control = noisy brain = marijuana use.
I don't think you could prove this, you'd need some way to quantify noise in the brain.
Interesting - that part about overactive brain, short attention span and insomnia matches ADHD very nicely.
In my case, since I began using stimulants to treat ADHD, my sugar cravings disappeared and weight stabilised. In the past I used to overeat, because it calmed me down.