Because most of them don't take drugs, but they do pay taxes. Most libertarians I know are straitlaced workaholics. What they hate is wasting law enforcement resources on victimless crimes when there are lots of thefts, robberies and rapes that could be investigated in greater depth instead of being dumped into cold case files. Some of the money saved from not imprisoning perpetrators of victimless crimes could be spent on longer sentences for thieves, robbers and rapists. Their perspective is that the state should not be shoehorned into the parental role - that role is best performed by each individual's actual parents.
Unfortunately too many parents are no more mature than their offspring. A few years back the 'state' of Missouri, had to restrict driver licenses for 16 year old new drivers graduated up to the age of 18. WHY because the stupid parents ignored and neglected to supervise their own children.
My exposure to dopers is limited but there is a wasteland of burned up minds creeping and crawling all across this land. There is no liberty in burning up ones brains. What is the cost of to tax payers caring for and coping with fried brains?