My guess is that it has something to do with
a) The need to censor guns (on TV) or have a movie rated higher (for cinema viewing) if it shows firearms, which means less people watching it*
b) The relatively strict gun control laws in South Korea, which means that what the shows/movies showed was quite accurate (http://www.koreatimes.co.kr/www/news/nation/2013/01/116_129474.html)
*I couldn't find any reference about this, actually. It's just my guess based on how they blurred stuff like knives (even kitchen knives!) or scissors on TV series.
