A show of force! Oh! Oh! Oh god! Yes! YESSS!
Look at it like a nuclear deterrent. You have it, I have it, we both know we wont use it. And it never was. I exclude Japan as I feel that reason differs from the long cold war that took place between USSR and USA
Theoretically the US and SK could respond in kind to what NK is doing. Even then, that is nit unreasonable. But they dont. As someone stated earlier there are skirmishes from NK at the DMZ, NK vessels have hit things. Despite the USA hate here, there is tolerance to what is a really really dangerous person. There have been instances that NK has done at sea and at the DMZ that war rules wise, could be taken as an act of war, allowing a full blown response. But that hasn't happened.