Is it really necessary for Korea to be against USA?  

It isnt neccessary
But you see SOuth korea is a member of NATO, so obviously US supports it
North korea being against south korea, sees US as a threat for itself
And due to the same reason it tested its nuclear missile
It isnt about george bush, it is about North korea fighting against south korea, and the NATO as a intermedator

America trys to extent its inluence too far. Some do't like it.

South Korea and USA always argue during trade agreements. They tried to open a free trade agreement between each other but it ended up beingput on hold because of rice. YES USA wanted rice to be a part of the agriculture agreement in the free trade talks but Korea feels that would hurt local farmers. The USA feels that South Korea should be more flexible and negotiate. South Korea thinks that the USA is being unreasonable. They also got touchy on talks involving medications.... USA is touchy on its big business medications. S.Korea wanted the meds because they want to renovate their health care. USA feels this plan they have will hurt the big business med companies. (Asinine) Such a vicious cycle they can't reach agreement. S. Korea is also touchy on textiles and automobiles. USA has S. Korean automobiles they are called KIA. I think that S. Korea wants their autos to gain popularity amongst their own people and feel that USA cars will threaten the Korean cars popularity. S. Korea seems to want it more one-sided then a fair trade amongst both parties. So far USA seems to be being the nice guy as they are taking Korea textiles, rice, autos etc. It isn't unreasonable for USA to want S. Korea to take USA products similar....

For those not too good with history: A short essay....
After WWII the country was split. North went with Communist Russia, South went with USA. North made a surprise attack take-over shortly after Russia put Kim Jong Il's father Kim Il Sung in power. USA and UK backed by UN support stopped this. S. Korea was able to stay democratic and out of the hands of the "supreme ruler." Most countries at the time were too busy licking their war wounds to care about Asia. So
South Korea was far more reliant on USA trade those days. As the years passed they were offered trade by several countries therefore making htem a successful part of world trade.
It has nothing to do with USA iconology or what someone said USA influence. USA INFLUENCE IS YOUR EXCUSE TO MAKE A UNNECASSARY KNOCK AT THE USA. Get some education so you don't have to keep falling back on your IGNORANT one liners.