The cost of the "drug" has dropped on the black market from $100 a kilo to $25 since its so readily available legally, and with growers based in the US, the demand has plummeted for cannabis that has to be smuggled into the country. The market is showing its efficacy very clearly (read a thorough breakdown on this topic from James Miller here). This is upsetting not only to the cartels, but also to the DEA.
The natural effect of legalization is for bootleggers and smugglers to die off, taking the violence that accompanies any illicit trade with them. You may remember a little thing called "alcohol prohibition," which is universally viewed as an idiotically bad idea that cost millions of dollars (and in 1920s dollars nonetheless!) and thousands of lives. Everyone agrees it was pointless and the country was better off when it ended than when it began. The insane war against marijuana (and other drugs) falls into the same category, yet