Jimmy Carter got a Nobel Peace prize for negotiating an agreement with North Korea. That agreement included removal of nukes from the DMZ. That's why NK is so feisty now. In addition to nuclear tipped artillery, we also had a contingent of Marines with suitcase nukes at the ready to halt an invasion of SK by NK. Those are gone too.
The SK military is big enough to fend for itself now. There is no need to leave our 37,000 troops in place.
Ahhh good Old Jimmy "The Peanut Farmer" Carter. He managed to do this while also giving away our interests in the Panama Canal and adjacent zones of interest and influence.
I'd guess it doesn't matter a whole lot if there are no longer any nukes on the DMZ when our nuclear sub fleet can lay just offshore with enough megatons to turn NK into glass. In any case, you're 100% correct: There is no viable reason to leave our guys there.
Jimmy Carter is in Venezula backing the votes for Chavez