Obviously US want to stir sh_t in Asia. They call their SK ally 'their national core interest in Asia'. It's obvious what that means.
It's obvious that SK knows also. But US is the few that are willing and able to protect them from NK. So they are willing to let the US to use them as a pawn in exchange for protection.
SK is the puppy for US as much as NK is the puppy for China. Just that it's much worse for China because NK is an unpredictable pit bull.