In the US State Department I'm not sure if it's explicitly given as official justification, but it's widely understood as the justification for the entire way things are organized.
Is it illegal when it's not the government doing it though? In what country, what laws make it illegal?
I'm pretty sure it's not illegal in my country but I dunno about the person I was responding to. I was just pointing out that it's SOP in some industries.
Rotating duty to prevent sympathizing with the locals is standard procedure for law enforcement agencies above the municipal level.