Because we are talking about lethal dose here.
Does texting messages kill? Well yes because they provoke accidents
Does talking to people kill? Yes because they distract you, hence killing you in road accidents...
Does gluten kill? Yes people are allergic to it lets outlaw it.
Plus it is very rare to be allergic to weed in comparison to gluten.
Drugs like alcohol and heroin have lethal doses, hence they can kill you by OVER dose