Knives being used as weapon fall under the "violence" category and would skyrocket the rating, making it impossible to air at that time -- if at all. Tattoos are considered highly offensive (you can't enter on jjimjilbangs if they're visible) and lighted cigarettes are a bad influence on young people. All of them fall under "harmful information to youths" and could get things banned or infuriate viewers and generate a lot of suing and apology demands.
Here's
the site of KCSC (Korea Communications Standards Commission) and an extract:
![eng_txt01.gif](https://www.kocsc.or.kr/eng/assets/img/contents/eng_txt01.gif)