You know, it's readily apparent that Trump doesn't understand how the military works.
This THAAD system that we have deployed to S. Korea are US developed systems that are operated by US personnel because it's a US system and has US classified systems built into it, so to keep our secrets safe, we deploy it to areas that we think it would be useful at.
If we were to sell it to S. Korea, they would have to send personnel over to the US to be trained on how to operate it, then take the system over, and we would have to hope they did it right. Even says so in the link...................
The deployment has also been a major topic of discussion in South Korea's ongoing presidential race. Front runner Moon Jae-in's top foreign policy adviser said Trump's suggestion that Seoul pay for Thaad would be an "impossible option".
"Even if we purchase THAAD, its main operation would be in the hands of the United States," said Kim Ki-jung, foreign policy adviser to Moon and professor at Seoul's Yonsei University. "So purchasing it would be an impossible option. That was our topic when we were considering the options."