Expat Pilots job in USA

[reaz]

I am sure this issue was addressed before somewhere.
Yet I need to clarify few specific points.

The job advertisements for USA indicates the following:

1. Must hold US Passport.
2. Must be a US Citizen.
3. Must be legally authorized to work in USA.
4. Few ads do not mention anything about Passport, Citizenship or work
authorization requirements.

Now my questions are:

1. Is it officially restricted to recruit expat pilots ( I meam, By Law)?
2. I know in other professions, expats are working in different visa categories and work authorizations.
3. Is there no expat Pilot currently working in USA?
4. If yes, then how the work permit or work authorization can be processed?

I will really appreciate responses having specific clues from someones own experiences and knowledge rather than general comments.

Thanks a lot.
Happy Holidays!