Earlier quoted context omitted.
I mean, is it really such a rare thing to do? I know that in Poland if you have any outstanding and overdue unpaid fines/alimonies/traffic tickets, you will be prevented from boarding a flight out of the country at any airport until those obligations are paid.
You're missing the point. People are being accused for crimes they (more than likely) didn't commit and denied basic human rights. They face a 99% conviction rate in the Chinese courts. https://www.theglobeandmail.com/world/article-china-charges-...
Japan also looks like it has a 99% conviction rate. Most countries have the ability to exit-ban foreigners visiting their country. It's not unique to China at all. US does it. Canada does it. South Korea does it.
I think the warning from the US Government is that, in their opinion, China will illegally detain you over made up charges, be careful.