In Western societies, once food goes into your mouth, nobody wants to see it or hear it. If you are raised that way, talking with the mouth full or making noises while eating gets on your nerves. I've been in both Korea and China for long periods and I just had to get accustomed to it and accept it. As H. Raquel puts it, it's their way of showing they enjoy the food. I can't see going someplace else and telling them how to eat. However, when my Korean and Chinese friends go to the U.S., I point out that they should change their ways. One Korean friend of mine made so many loud noises that he had half a restaurant looking at him when he first got to the U.S.

from my dad working in samsung for 10 years and many neighbors are also korean, for them chewing with their mouth open and that is a way to show that the food is good

In Belgium, I hear that it very very polite to put so much food into one's mouth that it can't be closed. Once a person's mouth is filled to overflowing, Belgians try to hum their national anthem. Koreans do a similar thing, yet they tend to try to grunt out jokes while their mouths are full.

It happens all over the world, in Australia it is bad manners to talk with your mouth full of food.

it does not only happen in korea, it also happens here in canada, and i am sure in the states aswell...it just depends on that person's ediquite(manners).