Robbery is not used by potheads to pay for their habits. That is the method used by junkies because coke and heroin are far more expensive than reefer and users develop thousand dollar a day habits. That is not the case with pot since there is only so much you can smoke in a day and it does not approach those numbers.
Some people can become unmotivated or at least motivated in ways not deemed socially acceptable but these are different issues from the LIE this article is pushing.
Pot does not dramatically increase violent tendencies and, in fact, acts completely in the opposite direction as your initial statement about motivation implies.
While it clearly is not a drug without negative unintended consequences it is also perhaps the least harmful of drugs particularly when compared to booze.
And use of any substance should never be a defense for murder. I don't really CARE if a killer was drunk, stoned on mj, hyper because of meth, nodding because of H. If he did it he should fry.
Pot inspires petty theft. I've seen it. Several instances. No, it doesn't normally inspire violence, it makes people lazy. But they will steal to support their habit if it's not too much trouble.