Yeah, it's not unexpected whatsoever. I'd actually be pretty surprised if he managed to enforce discipline without harsh punishments for those kinds of crimes, particularly in the face of an enemy like the Japanese samurai. I think it's not mentioned more to avoid upsetting modern sensibilities, or perhaps it's just so normal for the time period people don't really pay attention to it. It's said that he would personally fulfill some of his soldiers dying wishes so it's not like he only wielded the stick, he also offered the carrot. Actually, I've read parts of his war diaries and he was a pretty sensitive man for the time period, very upright and disciplined but also soft when it came to his family and whatnot. It's no wonder he's remained an icon for Koreans all this time (including me).