Exploitation in the drug market happens in the reverse of what you think. The target market is the first world, because the product has maximal value there.

The positive externalities are all gained by the producer countries. Traffickers and coca producers have even risen to become major politicians in places like Venezuela and Bolivia. Meanwhile, negative externalities -- crime, addiction, and so forth -- get distributed to the consumer countries.

The product is addictive, so you can't exactly be surprised that consumers continue to buy it. It is all too common in Latin America to hear people complain about the big bad USA, but that line of reasoning is casual and has its limitations.