CAVU, I can offer a little insight hopefully.
The Status of Forces Agreements (SOFA) with many of the nations that U.S. military contractors fly into require that the crews be U.S. citizens.
Yes, this is a requirement of many EU countries.
These U.S. contract carriers have no choice but to require that their crews be U.S. citizens.
SOFA agreements are similar to treaties. They are formal agreements between two nations that contain rules and requirements spelling out how the U.S. military (includes military contractors) shall operate within the host (foreign) country.
If you would like more information, e-mail me.
Cheers,