The US has 50,000 troops in South Korea defending them from North Korea why does the US station troops there? The US also stations troops and bases in Japan, Italy, Germany and the UK - oh also all over the Persian Gulf - Saudi Arabia, Kuwait, Qatar, Oman, UAE, etc. It is in the US's interest to have all those military garrisons because there are geo-political interests involved. Same in Israel - the US doesnt care about us but it is in their strategic interest to have a military strong Israel which helps project and protect US interests in this part of the world. You know, if there is a rogue country making weapons (Sudan) or a nuke plant (Syria, Iraq) the US is happy to have us take these things out. And, most significantly, and thank the good Lord, unlike all thoise other nations mentioned above, we dont ask for US bases and troops to be stationed here. The bottom line is the US - as any other nation does - acts in its own self interest which is fine and expected.
Iran's oil exports to South Korea quadruple to 400,000 barrels a day after sanctions lift (Reuters)