In the first case, one end of the law is to teach good behavior by punishing what is bad, and perhaps incentivizing what is good. Drugs are opposed to the objective human good, and gravely so, for one, because they cripple the effective exercise of reason. Any attack on reason of this kind is gravely immoral, and constitutes a direct assault on human flourishing and what is most essential to human beings. The result is the degradation of the human person and the degeneration of society. The common good suffers greatly, as you have described, and since the primary purpose of law and the governments that make law is to guard the common good, it falls to governments to criminalize and regulate drug availability and use.
In the second case, ask yourself why anyone takes drugs. Unhealthy curiosity. Boredom. Escapism. Mental illness. There is no legitimate reason for drug liberalization as there is no legitimate reason to take hard narcotics recreationally (I emphasize that word because there is a place for regulated use in the context of medicine under the principle of double effect). Liberalization lowers the barrier of entry by signaling that using narcotics is no big deal. And yet it is a big deal. The instructive dimension of the law, by penalizing use, helps counteract the foolishness and frailty of human beings that would bring ruin to such people and those around them. Libertarian anthropology does not stand a chance in the face of reality.
(Obviously, plenty of substances are psychoactive, and I do not propose a categorical ban on everything from cough medicine to cigars to Merlot to coffee. I am no teetotaler. We know what kind of drug use is problematic when we say “doing drugs”. Prudent regulation that takes into account a number of factors like the severity and nature of chronic and acute harm, how addictive something is and its potential for abuse, dosage, etc. It’s one thing to chew on a coca leaf, another to snort 30 mg of powder.)