This is a war on drug abuse. Proportionality has nothing to do with it, and expense is not an issue. It is a question of values - you are either for it or against it.
There are a whole bunch of wealthy, well connected people down at the Betty Ford Clinic being treated for alcohol and prescription drug addiction. By your reconing, either all of these people need to be locked up for drug abuse, or their doctors do for fraud for treating them for a non-existing condition. Which is it?
Human society consists of frail people who aspire to soemthing beyond them but fail. Some vice are unavoidably accepted as a result. The question is where to draw the circle. Alcohol is already within that circle, so with regard to it the question is of expulsion beyond the pale rather than status quo maintanace, which is what the war in drugs is trying to achive. These two tasks are widely different thus.
What you gave is a favorite among many non-example. The prescription drugs are not only within the circle by happenstance but by choice since they play a positive role. Some people will abuse them, as well as anything else, but that's another story altogether. MJ, in contrast, has no positive value other than some substitutable limit use in some cases. Legalization advocates do not suggest even that --- to make it a prescription drug. THis is a non-example.