In context of Victor and Cynthia Lu, the siblings father is accused of embezzling 1.4 billion. From memory, millions of assets was put under their names in form of US real estate making them legally culpable. More importantly:
>The Chinese Foreign Ministry defended the holding of the three family members, saying: “The people you mentioned all own legal and valid identity documents as Chinese citizens. Because they are suspected of economic crimes, they are restricted from exiting the country by the Chinese police in accordance with the law.”
https://www.nytimes.com/2018/11/25/us/politics/china-exit-ba...
China doesn't allow dual citizenship, Chinese nationals (including HKers) who naturalize abroad is suppose to renounce their Chinese citizenship. Many don't, choosing to travel on Chinese papers for expediency (no need for VISA) etc which subjects them to Chinese law. There's no extradition treaty to China which makes these dumb administration hacks backfire. The article also covered spouse of suspects arrested on suspicion of spying... which no shit justifies exit bans.
E: I think the greater story is that white expats in some countries no longer get preferential treatment due to trade war (US + Canada), and ex Chinese nationals are not safe from CPC reach just because they have a passport with no PRC extradition treaty, but especially if they're being dumb and travelling to China using Chinese papers they were suppose to renounce.