I think the general rule is not, but there exist exceptions. Foreigners (without greencards), however, are extremely difficult to hire due to export regulations

I think the general rule is not, but there exist exceptions. Foreigners (without greencards), however, are extremely difficult to hire due to export regulations (http://pmddtc.state.gov/regulations_laws/itar_official.html http://pmddtc.state.gov/regulations_laws/itar_official.html)