You might well be surprised at the level of front line/forward area withdrawl from the border areas along the DMZ by US troops, and replaced by young ROK troops over the last twenty years, and the increase in expenditures by the ROK for its own defense. And no, they do not sleep soundly with North Korea on their borders. And this is also not to say that the US does not commit considerable resources there, but things have in fact changed over the years.
point being...the American Taxpayer and military have been far more invested in others borders than our own...