It isn't equivalent of a JP Morgan banker in a Korean branch...

Someone who gets transferred by management to work in a Korean branch has to work to keep a (presumably) high paying job. Even if he criticises everything about the country, it still makes complete sense why that person would continue living in Korea.

An ESL teacher makes 2k-2.5k USD a month, which is less than most firms would pay in the US for a full-time job. To a Korean (or any rational person), there would be only two reasons why that person would continue staying in Korea: that person enjoys travelling and enjoys living in Korea, or that person needs employment that is unavailable to him in his or her home country. If an ESL proceeds to criticise Korea much more harshly than most Koreans would, especially if that person has been working for several years, it doesn't make any sense. Obviously once you start talking shit about the country, you aren't here because you want to, so you are here because you need employment that you cannot get at home (as I said, ESL salaries are less than American salaries), in which case you are criticising a foreign country that is providing you vital employment, and which criticisms extend far beyond just "IE sucks," but are more along the lines of "Koreans are a racist and rude people."

Having a college degree is not an indicator of anything. Like 70% of Koreans have college degrees, and many of them have college degrees from English speaking countries. There is a reason they choose not to teach ESL. And just to be clear, I'm not saying being an ESL teacher is a low profession. An ESL teacher who hates everything about Korea makes no sense to outsiders, but this type of person either constitutes the majority of ESL teachers or are the loudest, and therefore it is not a coincidence people associate ESL teachers with irrational bitterness.