Why is this so? By axiom?
Any parent will forcibly forbid her child to put any substance in his body. We make things illegal exactly because we recognize that a sufficiently large portion of the population is unable to handle a certain freedom. Now what exactly made you conclude that a sufficiently large percentage of the population can handle their use of drugs?
Making a person's diet a matter of law just goes against the values of many people. I'm well aware of the fact that some compounds lend themselves to addiction/habituation but so do many legal ones. You don't see people throwing winos and pack-a-day smokers in prison unless they otherwise break the law in a way that infringes the rights of someone else.
Likewise, if someone fails to use intoxicants or other recreational drugs responsibly and develops a debilitating habit, it makes more sense to do as we do for people who want to quit drinking or smoking, not as we do for people who rape and murder.
Making someting illegal does not mean you have to put people in prison. You do understand that we can forbid a drug and try to stop the sale of drugs, and still provide support for people who develop drug habits?
Funny that you mention cigarettes. I'm quite sure that cigarettes will be made illegal at some point in the future.
Any punishment oriented consequences are simply going to either be too harsh, or too lenient.
Does your scheme involve coercion of any kind? If not, then why is not working already? People are already free to seek help for their drug addiction.
According to the Declaration of Independence and the Bill of Rights, yes. The rights belong to the people, not the government. It's the government that gets the restricted set. One group of people shouldn't be able to tell another group what to do if it's not violating anyone's rights.
I'm not saying everyone can control themselves. I'm saying trying to prohibit is less effective and more destructive than education, rehab, and prevention. I for one do not want a society entirely engrossed in recreational drug use but prohibition is not the answer. When you prohibit heroine you get krokodil.