The South Koreans care about their sovereignty. Would you want our borders thrown wide open to commies? We don't have enough problems with the ones in office now.
It's commies light, in place of commies-heavy. That in itself is an improvement. In light of Skorea's endless appeasement of the thugs of the north, the China solution ought to be something they should swallow and too bad if they dont like it. They didn't do anything to fix the situation themselves so they can just eat it as China takes care of business.
Of course if they have a plan of their own to topple the Kim regime, so much the better. But they don't want to pay the cost, they think it will be too much and they want someone else to pay. They just don't get to always choose who that someone else will pay.
Eat it, South Korea. You haven't done jack. China will take out the trash since you did nothing.