There is a history of important Chinese personnel being kidnapped by e.g. the US when abroad. There is also a lot of talk in western countries about "banning Chinese [all presumed spies/propagandists/agents] from entering". On a good faith basis, one would think China banning people from leaving is a good thing that aligns with western desires, and should thus be applauded. So painting the policy as sinister tells me that the real desire is something entirely different.