And, if their best talent is anything like the other "leader in their field" people I know, they aren't particularly interested in becoming American citizens.
When you see the "illustrious" US government doing things like this, do you blame them? I don't.
If this should actually go on for longer there might be a danger that those employees just start their own companies in Europe or Asia.
This to me is a solid argument as to why they should ban it. US has a monopoly on this tech and it should stay that way.
If what they are planning on building is as important as they say any edge US can get it should take.
Having a large number of individuals who are not loyal to the country that provides this opportunity is a future threat the moment an advesary cuts a check.
If this is the nuclear bomb of our age would you want a large number of foreigners building it for you? If this action sticks I imagine every country will follow the same path and treat top AI scientist much like a top nuclear engineer.