Korea is the only country to pay to have American troops and bases. Subic Bay and Okinawa were American bases that the US paid a lease to the host country. American bases in Europe are funded by the US.
The US paid for the war in Iraq, Afghanistan and other countries. They are paying to protect Saudi Arabian oil fields. An article in the Wall Street Journal mentions the scope of US involvement since the oil field attacks.
The US pays for strategic military power in various regions of the world. In Korea, they have the THAAD anti-defense system. It is an early warning system for the US, in my opinion, and not of much use to Korea. There are enough missiles in North Korea to overwhelm the system and obliterate Korea many times over.
The only good thing he has done is stand up to China. It is a socialist country with undemocratic laws, extremely protectionist and unfair in trading. For example, to publish a video game in China, you need to get government approval, but they have not approved one single game from a foreign company. Only Chinese companies have gotten approval. So you have to be a minority partner in a joint venture with a Chinese company or go through a Chinese company. Either way, the business in China will be controlled by Chinese. There are seemingly endless examples of unfairness in all the other industries.
Korea can't afford to fight with China too hard. China accounts for around 42% of its trade surplus. Maybe Trump wants Korea to pay because of Korea's trade surplus, but if you follow that argument, he should be asking Japan to, at least, start to pay for US troops there.
Korea does not pay for the other countries that have military forces in Korea through the UN. Also the present situation with North Korea is not wholly due to South Korea's decisions and will, the US has the final world as the head of the military command in Korea during a war. Korea will take back command over its military forces even during wars in a few years, as agreed with the US.
The US military is all over the world and in dozens of countries, why is Korea the only country that helps pay for US forces in their country? And why does Korea have to pay more?
I don't think South Koreans are really scared by North Korea, militarily. We are more scared of China or Russia helping North Korea, and so is the US. That is one of the reasons that the US forces are still in Korea.
So, I think most people would see a sharing of the costs for US troops in Korea as logical. Asking for Korea to pay for the whole cost of US troops in Korea and related support operations in Guam and in the US ($ 4.7 billion), does not seem fair. It seems Trump is daring Korea to reject his demands and ask US troops to leave Korea.
(Updated 17 November - From an Reuters article, I found out that Japan also pays a share of the costs for US troops there and that the US wants them to pay 4 times more, around $ 8 billion for the costs. There are about 24,000 US troops in Korea and 54,000 in Japan.
