In Romania during the 70s and 80s the only people who would go to specialization/training in the West had spouse and kids that stayed behind. The punishment for defecting was that the defector would not see their wife/husband/kids ever again (well, it seemed to be for ever. Nobody expected the regime to crumble). AFAIK there were no labor camps in Romania.

With this in mind-I am quite sure NK is selecting the people win a similar fashion. I would not be surprised if the punishment for defection is more sinuster than just not seeing their spouse/kids ever again.

I admit this is a presumption, but I doubt they send people out without leverage of some sort.

dictatorships are the worst. but true, even professional athlete's when travelling defect etc

It's a threat. It doesn't have to be perfect. If the citizen does have a family or descendants, it will have a significant effective rate.

They can also select which citizens to send based on their ties. Much as a bank might size up collateral for a loan.