Strategic location for military presence...that about sums up WHY we do it. To be honest, we don't really "protect" South Korea anymore than their own soldiers do, the U.S. presence there pretty much keeps North Korea at bay 99% of the time (even now...notice how they've been playing games with industry vs. actually trying to "invade" from North to South).
And let's face it, North Korea isn't really why that military location is important right there in South Korea...it's China.
Balancing acts are expensive and redundant. And I don't agree with it 100%, but i do understand it.