>At least yet, no one can stop their top scientist to move to another country with the knowledge and just pick up their work in the new conutry.

They can and do do this routinely. Many individuals get marked and regularly go through additional screening if their travel plans raise flags. This isn't even unique to the US... most Western nations do the same. If there is a serious brain drain risk, the US government can easily go all out and have the whole company put on the no-fly list.

>At least yet, no one can stop their top scientist to move to another country

Let's hope so, because I am not so certain.