Yes, the whole issue of drugs is a troubling one to libertarians(like myself) and those who do responsibly use drugs for recreational use. The problem I see with their ever being legalized though, is that the government has had a policy since the late 19th century of government sponsored social betterment(a term I derived from an article I read on Thomas Paine).
Because of the the rise of the middle class, who wielded much greater control over society and government policy than non-nobility in the past, there was a realization that many things were wrong with modern urban life that affected the middle class as well as the poor classes that nobody gave a shit about. They also realized the poor classes, if they're lot were improved by government action, could become more upstanding citizens, and thus lower the danger of a worker uprising against middle and upper classes.
The government thus has in place a policy of supporting popular laws and programs that sate public interest by showing that what it is doing is the interest of the people, and that people should realize that life is better for these laws and programs being in place, making narcotics illegal one of them. Supposedly, if drugs legal, they would break apart families, lower the productivity of people, and generally cause a mess of pacifist society.
While the people are convinced of this idea, drugs will never be legal, and while people are convinced that few people are responsible and intelligent enough to control their impulses enough to lower the risk of drug use, they will support laws making drugs illegal.
Other programs in place include the requirement of sanitation in cities, public housing, and a variety of laws and programs designed to keep people from lowering the general quality of life as a result of their poverty and perhaps weakness of character. This is primarily why drugs are illegal, the belief that if they were legal, everyone would use them and then everyone would be dragging down the quality of life and such. Also gambling and prostitution are made illegal for the same reasons, as was Alcohol during prohibition.
Incidently, the police force's aim in society was shifted to appeal to these sentiments and its these reasons that most people accept the pressence of police in our every day lives.