There is no catch. Korea is growing fast economically and the need to learn English has become vital, so English teaching jobs in Korea is in high demand, as Korea wants to be included in the global business world. Korean schools realized they needed to offer incentives and great benefits like the free flight and accommodation to attract foreigners. The high availability of jobs makes it a competitive for schools to find individuals wanting to become an English teacher in Korea.
In order to enter good universities, students must do well in English. In order to do business internationally, people must be able to work and speak in English. Furthermore, they want to learn to speak properly, with correct accent and pronunciation. So, they require native English teachers such as you.
Most people are not willing to take a year off and move to a foreign country for many different reasons. It takes an adventurous spirit and a lot of guts. Teachers, especially from North America are in great demand. For you, it may seem like the adventure of a lifetime, but for most people, it is very scary to leave home for a year.