Yes, the US is well behaved, as far as these things go, mapping the mines, making them auto-deactivate and detectable. But the problem is, the US is seen as a beacon of morality. Maybe less so than it used to be, but the rest of the world still has fairly high expectations. If the US is using landmines, other countries can us this as a moral justification to do the same.
The US doesn't need to use them in Korea. They can cede responsibilities for those to South Korea. Pointing to what South Korea does is a far less compelling moral argument.
However, on the matter of Korea, the proposed alternative is autonomous guns. Unlike mines, these things don't hide, so, at the very least, we know if all of them are accounted for. But these could be banned by an overly broad anti-robot rule.