Sorry, but that is bullshit conspiracy theory nonsense.
The US has no need for "Korea" for "security" in the region. We already have a much larger and more capable ally in Japan, which has much better facilities for the military than Korea could ever offer. As well as Guam, once again much better than Korea.
In reality, I can think of few places well suited for a "military presence" than a peninsula. Hell, this can be seen at the very start of the Korean War, where the US Military was in much the same situation that the UK-French forces were at the start of WWII at Dunkirk. But with almost 100,000 US soldiers trapped at Pusan, there was never any hope of evacuating them so they had to dig in then fight their way back out. Looking back at many of the battles in modern history, the absolutely worst place you want to be in such a situation is a freaking peninsula! Especially one without any particularly good ports that can be fortified to make a withdrawal from.
This is a fact, there is absolutely nothing the US gains in Korea that they do not get much better from Japan. We only remain because South Korea is a key ally, and we are obligated to support and defend them. And much like the vast majority of US bases in Germany, once unification happened the US simply smiled and walked away.