Problem: Afghanistan's defense minister, a critical figure in the government, is apparently involved in drug trafficking.
Solution: Well, he's on our side, so let's not do anything about it.
This little bit of cynical realpolitik actually makes more sense than being righteous about it. After all, every dollar in the drug trade that goes through the government is one dollar that is not going through the Taliban. So it's really just solidifying market share of the only export Afghanistan seems capable of producing... right?