“If drugs can’t be kept out of prisons, how can you possibly keep them out of a mostly free society?”
Drug addiction (any addiction), is a matter of personality type. It is not the drug or activity that creates the addiction. It is the personality type that finds something to be addicted to. This explains why some people can have a drink, gamble, or take a toke, and be fine, while others cannot stop.
This Psychology Today article explains how 10%-15% of a population has addiction tendencies. Even the rampant drug use by Vietnam vets during the war returned to the same percentage levels once back in the States.