You want a rational basis? Fine.
Your brain was designed to certain specifications by the manufacturer. There are certain chemicals that can adversely affect the brain and, by a side effect of their use, produce an euphoric sensation.
Naturally this feels pleasurably, however it does alter the brains natural function and eventually, addiction sets in.
You no longer have the same brain chemistry, you lose memory, you lose higher thought process. You become a danger to others.
You become a danger to others because of your chemical dependence.
Let's be realistic, even if drugs were legal, they would not be free. More people would be open to using them, nothing productive would get done, crime would increase in order to pay for the legal "fix". Through illegalization, mainstream drug use is highly marginalized. It exists among all levels of society, but is kept in the minority. Most people value their freedom, lives, and property more than they value getting high. Those that don't are caught and prosecuted. This marginalization keeps the standards high. I would not suspect that most people want surgeons, air traffic controllers, pilots, or other such people who enjoy recreational drug use that does affect motor function, cognitive thought, and rational behavior.
This includes anything psychotropic, including that "harmless" drug marijuana. The effects of marijuana last for 90 days after initial use. This is compounded by regular use. This causes a loss of short term memory, a highly important brain function. It also causes some spatial disorientation and slowed reaction.
Would you want an airline pilot with slowed reaction? The fact is, most humans can react only so fast to begin with, and there are still plane crashes. With even slower reaction than normal that increases the risk.
Most of society, which I normally show disdain for, has agreed that drugs are a scourge and should be highly discouraged and illegalized. I agree with this. I would go a step further and say that once you alter your brain in such a fashion, you are no longer good to anyone. You cannot be trusted. You have displayed weakness in the name of recreation.
By killing all of the drug traffickers and users, we send a message. It will not be tolerated.