It has to do with if you consider it moral to take recreational drugs.
Many religious and political groups in the USA consider it immoral and
have tried to legislate their moral beliefs by making recreational drugs
illegal. Of course, by now, there are many commercial interests (privatized
prisons for instance) who have a lot to loose as well.
Though maybe it's just this:
"If You’re Against Recreational Drug Use, There’s A Good Chance You’re Against Recreational Sex."
While many items predict to some extent whether people are opposed to
recreational drugs, the most closely related predictors are people's views on
sexual promiscuity. While people who are more religious and those who are more
politically conservative do tend to oppose recreational drugs, in both study
samples the predictive power of these religious and ideological items was
reduced nearly to zero by controlling for items tracking attitudes toward sexual
"This provides evidence that views on sex and views on drugs are very closely
related," said Kurzban, associate professor in the Department of Psychology and
director of the Pennsylvania Laboratory for Experimental Evolutionary Psychology
at Penn. "If you were to measure people's political ideology, religiosity and
personality characteristics, you can predict to some degree how people feel
about recreational drugs. But if, instead, you just measure how people feel
about casual sex, and ignore the abstract items, the predictions about people's
views on drugs in fact become quite a bit better."
more here: http://www.physorg.com/news195884892.html