It relates to how drugs get into the country, and what drugs get into the country. In the case of the U.S., the Mexico border is a huge problem. It probably didn't help that the CIA was selling automatic weapons to the cartels, either...

It's much easier to catch someone smuggling drugs through an airport than it is stopping someone from walking over an imaginary line that's thousands of kilometres long. So less drugs in circulation, which would have an effect on death rates.

Koreans have boats, and a very large coastline...

Sure, but their closest marine neighbor is also very hard on drugs and drugs dealers (Japan).

Given that drugs are grown/cultivated elsewhere and smuggled in, and smuggling them is close to a $100 billion per year business, very.