Having an alliance does not equal having to station our troops there and immediately put them in harm's way, not to mention the costs involved.

Not to mention there's no real reason for China to attack SK. Attacking Taiwan (which China considers its territory) is one thing. Invading a sovereign nation that China itself recognizes would be something completely different.

Let's put it this way. We don't have troops in Taiwan. And yet China has not invaded, because the threat of the US assisting Taiwan militarily is enough deterrence. It'll be the same with SK.