You do know that Korea was artificially split in half due to the ongoing war, don't you?
https://en.wikipedia.org/wiki/Division_of_Korea
Germany was also split, and was reunited. I don't believe it's necessary to explain why.
In fact, re-read that comment, because what was said pretty much advocated policing absolutely nobody.
But, okay, let's all argue. You're right. North Korea isn't isolated from the world at all, and its citizens pretty much go anywhere they want, do whatever they want, and come back home whenever they're done doing as they please.
But I wonder: if the typical North Korean citizen were unhappy (and we know they aren't), where would they go, and what would they do, if this hypothetical repressive lid (which doesn't exist) were suddenly lifted, and the pressure of a (normal) subculture were relaxed?