Marijuana is no more harmful to the individual than smoking cigarettes or drinking alcohol; in fact, it may be significantly less harmful than cigarettes in terms of the frequency and manner in which users typically consume both substances. Whereas marijuana may be consumed irregularly or once in a while by many of its users, virtually everybody who smokes cigarettes does so on a daily basis, usually consuming ten or twenty cigarettes or more every day. In all likelihood, the average cigarette smoker is at much greater risk from smoking than the average marijuana smoker is.

In principle, government could justify prohibition of marijuana, alcohol, and tobacco uniformly and without distinction without necessarily violating any logical or moral concepts. Admittedly, ingesting any form of smoke into the lungs unnecessarily is medically harmful and unnecessary. Especially since the cost of medical insurance and un-reimbursed medical expenses from public resources is such a tremendous economic issue, it would not necessarily be unjustified for government to prohibit the commercial sale or private use of all three substances for the public good and individual welfare.

Likewise, legislating behaviors associated with the consumption of intoxicating...

In that case, it is not the consumption of the substance that is the issue but the irresponsibility of driving while under the influence (for the most common example) that is the basis of criminal culpability. In that respect, any form of consumption of intoxicating substances is fairly and properly legislated for the public good.
What is unconscionable in current American law is that both alcohol and tobacco are legally sold and taxed while marijuana use is subject to criminal penalties and the horrible consequences that often flow from criminal prosecution and punishment. In fact, under current law, it is perfectly legal to consume alcohol before driving provided one does not exceed a statutory limit for blood alcohol percentage. That is notwithstanding the known fact that any amount of alcohol reduces reaction time and increases the risk of vehicular accidents which present a danger to anybody sharing the road with someone who recently consumed a few glasses of wine with a meal or a cocktail or two after work even where his or her blood alcohol remains under the legal limit.

On the other hand, in many states, the private use of any marijuana even in the complete privacy of one's home while doing nothing more dangerous than watching television or reading remains a crime. It is that unjustifiable difference in the way that marijuana is treated at law that is unconscionable, irrespective of the general right of government to impose paternalistic legislation.