If you were running a country, would you want your brightest and best people leaving and never returning?

A large proportion of chinese people have to go back to China after being educated, or china will discourage them from leaving in the first place, since it wouldn't be in the national interest.

The US does similar stuff - through their worldwide taxation scheme, they discourage citizens from earning lots of money abroad. They're pretty much saying "if you're successful, we want you to bring those profits back to America".