I work in a global company. As an interviewer, I don't even know if the person I interview is a US citizen. I don't even know if they live in the US or another country, and I'm probably not allowed to ask. I just rate their performance. And everybody is paid the same.
I think the situation is quite simple. Companies want the best candidates they can get, and the larger the recruitment pool, the better.
If tomorrow you were to tell (let say) Google they now need to hire only US citizens, would they lower their hiring bar, or would they expand their Shanghai or Zurich offices? Are US citizens entitled to work at Google more than others for the sole reason they have a particular citizenship?