Well, the article pointed to three different sources of drug violence - violence from intoxication, violence from getting money for drugs, and violence from selling drugs. They need to be dealt with as separate issues.
Violence from intoxication depends on the drug. Alcohol is probably the worst, followed by amphetamines and cocaine. Nobody wants to break a nose because they're high on pot. So the laws don't line up with violent potential.
Violence from getting money for drugs is because drugs are expensive, and addicts tend to be social outcasts. It's also incidental to property crime in general.
Violence from drug sales is 100% the result of prohibition. During alcohol Prohibition, alcohol business crime was widespread. Today, it's nonexistent. Likewise, there's virtually no violence in the field of loosely handled prescription drugs. If drugs (not just marijuana, but hard drugs like cocaine and heroin) were broadly legalized and readily available in a safe and competitive market, violence from both the drug trade and the cost of drug addiction would disappear.
Which makes one ask, "Why do we do this?", when it makes so little sense. Then one might answer, "Is racism a root cause of the war on drugs?", and a whole bunch of people would howl with indignation.