NK regime itself is a large-scale METH and heroin producer. They converted their pharmaceutical factories into giant METH labs. Once large quantities of METH were exported to outside(predominantly China) to earn hard cash for the regime. Now that China and other countries crack down on METH smuggling, export became difficult, but they still have to generate revenue. They started market METH inside NK. METH trade is conducted by power-that-be of N. Korea. Lately, NK regime started to crack down on METH. They could go after end-users and small-time pushers but I doubt that they can touch big fish, unless the big fish is the main target of Kim duo's purge drive. NK regime is broke(even though Kim Jong-il isn't) and they need cash to sustain the regime. This is not going to disappear soon, and probably become one of the main factors to bring down the regime.
NK regime actively raised this monster by making MEH production as a major export industry of N. Korea. Now it is having a blow-back.