The United States have some projects in South Korea that would be negatively impacted by cooperation with the North:
- A ballistic missile defence (BMD) system in South Korea. Its defence area allow to use it against Russia and China as well. It is an important part of U.S. "global first strike" concept. Those BMD in Europe, Korea and other US allies should intercept possible return strike. They are (mostly) placed near opponent's borders to intercept missiles on their launch, which is much more simple, than intercepting near US borders.
- Natural gas supplies to South Korea. There is a project of building trans-Korean gas pipeline from Russia. And now, when Russia-China pipeline is build, this project is much more easy to build. Obviously, this will end US supplies to South Korea, because pipeline gas is much more cheap than naval-supplied. US is trying to prevent European situation where russian natural gas pipeline easily beats US naval supplies.
These are only available if there are tensions between the Koreas.
But is there something good, some advantages for the U.S.? I think that the U.S. could block negotiations (for example, starting military exercises, which are definitely provoke North Korea to start missile launches) if there were no advantages. But negotiations took place. So, what are the advantages to the U.S.? I have thought about it, but cannot imagine any.
I am aware of Why does the US engage with North Korea? My question is not why the US engages with North Korea on some level. My question is why the US does not attempt to prevent relations between the North and South. What advantages are there that outweigh the disadvantages that I have listed?