It is not because we need to subsidize their defense per se, to cite one argument which some non-interventionist critics have attacked. It is so, when North Korea behaves in a ridiculous manner, the South can respond (not respond) with great restraint. What we are subsidizing is a) a feeling of security, and b) not building nuclear weapons in response. We do something broadly similar for Japan.
The potential problem is when the same U.S. acts which produce a feeling of security in South Koreans produce a feeling of insecurity in North Korean leaders. And the broader game we are playing, with numerous allies, means we might end up pushing some individual confrontations beyond an optimal point (e.g., how would Israel respond with Iran if we wavered on South Korea?) Might we have to overinvest in the South Korean feeling of security — from a strictly Korean peninsula point of view — to keep Japan, Israel, Taiwan, the Saudis, and others “in line”?
It would be good if the North Korean leadership would read this blog post, as they would then realize that what to their eyes appears to be American “overstepping” is done for the sake of other audiences. It is problematic for the American government to itself communicate this point. Imagine announcing “we don’t stand by South Korea as much as it appears, we are just doing this because Israel faces a signal extraction problem and we can somewhat sway their inference toward relaxing about their own security situation.”
It would be bad if the Saudi leadership would read this blog post (or understand this to begin with). The American government would then have to produce a feeling of security for South Korea all the more.