I think they know they are doing nothing of value and want to try to justify their jobs by detaining people occasionally regardless if there is a real need to or not. They, like regular cops, have to meet a quota to justify their paychecks.
And at which point did they trade in their human decency, civility, and morality in order to behave like immoral assholes?
When they realized they could get paid to do so. That's all it takes for lots of people, money. But they were probably already assholes, so this jobs pays them for being themselves.