I am a firm opponent of the drug war. However...it's not wise to argue against a topic without being able to articulate the best case from the opposition. Correct me if I'm wrong.
People vary across many dimensions. Among the measured, well known, stable factors on which people vary are:
Openness to Experience
Conscientiousness
Patience (ability to delay gratification)
IQ
Given the reality of bell curves, we can construct (or find in the wild) a person with average patience, conscientiousness, and IQ. In the event that said person encounters an addictive substance, be it Marijuana, Cocaine, or Meth, that person's ability to live a productive, happy life will be seriously impaired. As a means by which to support peoples' long term happiness, we as a society need to keep drugs illegal, so as to prevent long-term-bad-for-the-individual actions.
The normal response to the above is that while it might be true, it applies just as well to Tobacco, Alcohol, and in the limit case, maybe even sugar. Of course this is true. However, practicality intervenes. The argument, as presented above, applies relatively equally well to both tobacco and alcohol. In the case of tobacco, we as a society are moving in the direction of illegalizing it, even though to a significant extent, our country was founded using Tobacco profits. This correctly brings tobacco it into line with our other drug policy. In the case of alcohol, while it's hard to make any case at all suggesting that alcohol is either less damaging or less addictive than other substances...we've tried alcohol prohibition, and given the fact that the average person in our culture drinks...it is impracticable. It would take a 100 year culture war, and we've yet to find a way to do that effectively. Not only that, the abolitionists acted too early, failed, and we'd have to deal with the fallout from their failure. Not worth it, even if theoretically wise. Illegal drugs, on the other hand, are consumed regularly by very few people...with the exception of cannabis, drug usage is very low through the country. Hence... while we'd like to prohibit alcohol as well, our 2nd best, and only realistic preference is to just prohibit all the other drugs.
The average person (or perhaps slightly subaverage on one of the 3 big success factors: conscientiosness, patience, IQ...which is true of
most people), when exposed to drugs, has a worse life. To the extent that we socially make choices, it is 100% appropriate to tilt the playing field so that said average person makes the choice that they would prefer in the long-term, given all the real consequences, and including Kahnemanian limitations. Illegalize the stuff for a Sunstein/Thaler nudge. It's sad that we don't have any more effective tools than outright prohibition for the purpose of said nudge...but we don't.
In addition, it's well known that the average low IQ, low Patience, low Conscientiousness person...the same person that is likely to be arrested (caught) for drug use is also the person most likely to commit more serious crimes. While granted the fidelity of the process is imperfect, and some largely innocent folks are caught in the net as well, the drug war effectively serves as pre-emptive crime detection. Arrest those people who are otherwise likely to be serious criminals, and crime will fall.
The drug war is therefore the best policy for the average citizen. It unfortunately sweeps up a few folks who are otherwise harmless...but as a cost of making a safer, more productive society, you don't get much better.