The imports from DPRK are not the problem, its the exports to DPRK. China is their life-line for domestic goods, fuel, food - most of it appropriated for the military, literally propping up this regime. As the Chinese themselves wrote, hundreds of years ago; "To catch a Thief, first catch his King". But, one must do so with nuance and subtlety, so China does not feel embarrassed. A reminder of what China stands to lose economically is a good tactic.