It might cause a few deaths but it also sustains the multi billion dollar prison industry and employs well over 1 million people in the US alone
None of those jobs help the economy. Why should people be employed in occupations that have no benefit to society whatever and are in fact detrimental to society?
The government profits from illegal drugs even more than drug cartels do.
Colorado's pot legalization and the multi-billion dollar alcohol industry shows that governments profit a lot more from legal, regulated drugs than outlawing them.
I've known drug addicts, and the WHO is also right about compulsory addiction treatment; compulsory treatment flat out doesn't work. The addict has to want to stop, and it's very hard even when they want to. Alcoholics and other drug addicts relapse more often than not after treatment.
However, should they ever invent the fictional drug in the novel I'm writing (see my journal, the first crude draft is being posted there) I sure hope it's not legal!