They still are a client state of China. China will not take kindly to the usurper messing around in their back yard. Quantity still has a quality of its own.
Ya know, come the right circumstances China may choose to let NK go a step too far, creating a sinkhole for the USA there while China springs another “none of your business” attack elsewhere.
Some megalomaniacs realize flunkies make great disposable distraction targets in a pinch.