They probably hoped that the remaining 15% was just enough to keep the workers from defecting. That combined with the threat to brutally torture and kill their family that remained in North Korea were probably pretty effective motivators to stay loyal to the regime.

Maybe it's not so bad in North Korea? :)

I doubt these North Koreans are getting hired due to wage disparities (these are roles supposed to be in the US where they have a contact person in the US), more like they have perfected the interview process as the most important thing in their life.