I've heard of this before, and am also somewhat familiar with use of cannabis to alleviate symptoms of opiate withdrawl.
One of the most common reasons people become habitual in terms of other dependencies is self-medication, and anxiety/social anxiety is classic. Now, we all know how effective cannabis can be against anxiety, so it stands to reason that if you lessen the symptoms of the disorder that incites people to seek relief by using more harmful drugs, you can also diminish their use of those drugs.