If people are allowed to buy, sell, and even grow their own drugs legally they will no longer have to participate in a black market that often benefits Mexican drug cartels.
My previous question is actually to find out what kind of measurement, precaution or whatever it is, to make sure that Prop19 will cripple drug cartels, without creating another problem.
Moreover the profit margins go way down because you are no longer covering the risk of arrest to those growing and supplying the drug.