Shouldn't every country be barring people from leaving the country if they've been charged with a crime? At least if there's a good chance they will flee justice.
This seems like a side issue from the question of whether the charges are legitimate.
The Manus founders had already left China. They were called back and went willingly, because if you don't go back, then China disappears your family.
It's extremely common even without a crime. US block or cancel people with extremely small child support debts (I think like $1000 which is a single missed payment for middle class person) and people with moderate tax debts (I think about $25,000) for instance from getting a passport.
Yes, everyone country does this. You can be barred from travel in a wide range of other circumstances in many other countries.
Every person has a nationalistic solipsism that renders them incapable of understanding events that occur outside of their own country. China and the US are two countries where this tends to be most severe, people outside these countries seem to believe they possess a profound and innate understanding of events there that renders them capable of offering a complete opinion (and, in reality, that opinion will almost always be entirely self-referential, 20% of the comments on this thread seem to be talking about the US).
At a high-level, the characterization of China as a lawless dictatorship is undermined somewhat by the higher levels of crime in almost every other country. You will see this interpretation of China from people in the US who live in places where there are constant, visible signs of crime.