Defending the occupation of Jail Guard. Humans look for patterns in everything. This behavior is fairly well documented at this point.
Generally sterotypes are based on observations over time. They aren't 100% accurate but usually stick fairly close to the median.

Jail guards do not have a reputation for being kind, gentle souls. Historically we have lived day in and day out with some of the worst people our society has produced. Being kind and gentle will get you manipulated or hurt at best or killed at worst.
Historically we have also been out numbered.
Have to establish ourselves as the Alpha dogs. There different ways to do this. Depending on a persons training o

Being "hard", consistent and unyielding is, imo, the best way to go about your duties as a guard.