As part of a '70's memorandum of understanding between the US and ROK, giving the ROK access to some US missile technologies, they agreed to limit the range of their missiles, about 180km. Recently, the MoU has been modified, allowing the ROK to design and deploy a ballistic missile that can hit any part of the DPRK.
And, to reply to dj245's comment as to exactly who's to blame for tensions on the DMZ, tension is the very thing that gives the DPRK government legitimacy. They deploy tension whenever they feel the political and/or economic need. It is possible that if the US unilaterally withdrew its forces, the result would be nothing. But, that's a guess. What's known is that with US forces in harm's way, the DPRK's military commission has to take the possibility of massive US intervention into account.