I had the same thoughts. Seems like we need to establish a M.A.D. (Mutual Assured Destruction) policy here with regard to the two Koreas. It worked with the USSR, and with Reagan's leadership, it eventually bankrupted the Soviets.
Unfortunately, it's only a half a threat. The Korean peninsula is not a big place - and the prevailing winds blow south. There is no way it could be used by the South even if it had one without contaminating itself.