Why do you call them ? They are an ally and they are a great country and people. It's in everyone's best interest that we are friends and the foreign aid that we have sent them has been some of the best spent money in the history of the world. Don't believe me? Go there and spend 18 months (or longer, if you wish). It's wonderful. And, as a member of the international community, they do their part:
http://en.wikipedia.org/wiki/South_korean_military
Go to the bottom and check out their oversees deplyments. The Koreans are doing their part.
They are great allies because of their strategic position close to Communist China, North Korea and formerly the USSR as well as Iran. It's just a base to station troops and weapons so they are ready to fight these countries or intimidate them with their presence, the 'trade' is compensation for this. Why do you think George Bush started sucking up to the likes of Poland and the Czech Republic, how do they differ from any other country? It was to put intelligence gathering equipment there (would have been able to collect information about all movements in Russian airspace up to the Urals mountains which is the whole of European Russia) along with missile launchers as part of the missile defense plan, both Russia and Iran could be reached from there. It's no different to someone making a friend who has connections that can be useful to them, maybe they wanna borrow their car or they can get them a job or they got a hot ****** and so on, International relations are no different (Turkey for use of its airspace during the Iraq war etc).