I'm sure we both agreed that smoking tobacco should not be encouraged, yet we do not threaten tobacco users with arrest and imprisonment. Now consider dangerous sports, such as mountain climbing. - The death rate on Mount Everest is about one in ten of those who make it successfully, which is a vastly higher mortality rate than just about any drug used at present in a recreational manner.
Maybe you believe that it's immoral to use a certain drug, but if you also wish such acts to become/remain criminalized then surely you also have to accept responsibility for the dire unintended consequences of such folly, when these certain plants/concoctions/drugs are then sold only by criminals and terrorists; when the huge black-market profits are used to bribe and threaten law enforcement officials when the availability and usage rates tend to go up, not down; when our prisons become filled to capacity with easily replaced vendors and smugglers - this list is actually endless!
Add to all that the fact that law enforcement and rehabilitation are mutually exclusive. Would alcoholics seek help for their illness if doing so were tantamount to confessing to criminal activity? Likewise, would putting every incorrigible alcoholic behind bars and saddling them with criminal records prove cost-effective? I think most of us already know the answer.