Heroine is a prime example. Whilst the argument could be given for "Educated use" you must take into account that the greater population at large will not educate themselves, and will come to harm - mainly through addiction.
Cigarettes are a nice example. We spend vast quantities of money on health care for people who smoke - the information is there, everybody knows that it causes cancer, yet huge amounts of people do it. We would be better off as a planet if everyone just stopped.
Whilst incarceration clearly is not the correct response to addicts, some form of regulation is needed to protect idiots from themselves.