Nicholas Kristof says,

I'd like to see American bases remain in South Korea, but right now we're taken for granted. We can't want to protect South Koreans more than they want to be protected. So we need to convey the message: "Look, if you don't want us, we're out of here. Why should we pay $3 billion a year, not counting the cost of the tanks and the planes, to keep 37,000 troops here so Korean ingrates can slap them around on the subways?"