I think they do mean harm, they want to distress the people because they know that people in distress tend to self-accuse. They go as far as they can on the spectrum from verbal abuse to torture.

Moreover 1) there is absolutely no need to behave that way even to a criminal 2) people on the international zone of an airport have not yet had the opportunity to commit any local crime, since they are not yet in the jurisdiction, the entire premise is flawed if they are not returning. And if they are returning and accused of a crime, there is a different system in a different room for them.