I recommend people read The Politics of Heroin and A Legacy of Ashes. Both great books.
Basically, The US makes drugs illegal and then enforces embargoes against other countries so they comply. Drugs have a very inelastic demand so this doesn't actually stop people from using them, but it DOES make drugs an extremely profitable enterprise.
Keeping drugs illegal has at least one massive 'benefit' -- if law enforcement grants anyone immunity and gives them permission to deal in drugs, they are essentially immediately granted billions of dollars. It's really not hard to grow poppies or cook meth or truck packages between cities. Nothing about that is intellectually difficult.
Consider the following: If the head of the CIA told you that you had total immunity and could deal in drugs all you wanted, how much money could you make? The answer, for any idiot in the world, is literally billions of dollars per year within just a few years.
This becomes enormously useful when we want to implement regime change around the world or otherwise provide funding to certain elements without directly giving them money.
Someone comes to the CIA and says they want to overthrow the government of Nicaragua but don't have enough money? Just let them deal drugs. [Contras]
Someone comes to the CIA and says they want to help fight against the soviets in Afghanistan but don't have enough money? Just let them deal drugs. [Osama bin Laden, Hekmatyar, various other Afghan groups, and Pakistan's ISI]
Someone comes to the CIA and says they want to fight against communist-backed forces in Laos but don't have enough money? Just let them deal drugs. They run out of adult soldiers and have to start using children? That's fine, give them the go-ahead. [Vang Pao]
The illegality of drugs is integral to our foreign policy strategy.
To my understanding, until about a hundred years ago, we didn't mess with other countries. Remember our policy of "isolationism"? It's hard to, these days, but that was once a core part of our national identity.
But since the end of WWII, the US has overthrown a sovereign foreign government, attempted to overthrow them, or attempted to overthrow them through support to a third-party, almost once per year. Literally about 50 times since then. Our intervention with some countries is absolutely brutal. We installed the Iraqi governments both immediately before and after Saddam's. You think Iran hates us because of our freedom? I think they hate us because we refuse to respect theirs.
Understand that the OSS/CIA was created because we had no idea what was going on during WWII. Prior to WII, we did not have a foreign intelligence agency. During WWII, we did, but it was an Intelligence agency, not a paramilitary or interventionist organization. Then everything went to crap and now we are the most hated country in the entire world.
Drugs let us weaken and overthrow regimes without doing anything. Suddenly, _inaction_ is sufficient. Just don't arrest that guy or any of his friends, and don't investigate their activities.