As they have the power to mistreat humans due to the combines rule in order to control the population. Also, they are actually evil.
Plus, if what Barney said is true, then they are in fact obligated to use physical force on citizens.
Although when Barney says "I'm way behind on my beating quota" then he's smiling so in fact that statement might be merely a joke.
As shown in the Stanford prison experiment (look it up if you're interested) people put in a position of power over other humans, especially when part of a group, quickly start abusing this power and commiting acts of violence against them beyond what they would have done normally.