Actually it goes against your point. How is someone using/buying pot different than someone using/buying a hammer or anything else?
Pretty simply because buying items from criminal enterprises furthers their ability to perform criminal acts. Hammer manufacturers are not murdering each other's hammer salesmen for a slice of that juicy hardware market. People are not being tortured to death for debts owed to hammer manufacturers. Drug cartels and gangs do so routinely, and drug money fuels that crime.
The "but an item can be misused" is a seperate and distinct argument that is just a red herring to this one. The drug trade creates crime other than drug use simply by it's existence in it's current form giving economic power to criminals. The hardware industry does not.