Yes, North Korean weapons of mass destruction pose a threat to the United States, because they are an enemy of the United States. North Korea uses the United State as a target, in order to trick its people into hating the United States. The leaders of North Korea are crazy enough to kill their own relatives for new reason, so they'd probably nuke the United States if they thought they could pull it off.
The North Korean government is controlled by insane tyrants who run what is arguably the most closed and repressive society on the planet. For such a government to have access to nuclear weapons is intolerable and they certainly pose a threat to the United States. For example, using a submarine, it might be possible to smuggle a nuclear weapon onto American soil without even having missiles that can reach this far. Also, nuclear warheads can already easily reach South Korea, where many U.S. Servicemembers are stationed. Being drawn into a nuclear conflict in Asia would be disastrous for us, even if the homeland were not targeted by such weapons.