I'm not sure if you are being purposefully obtuse. Of course they can. They can get any job they want, and they do. Americans who graduated with a PhD from any of those institutions will never have trouble getting a job on whatever they please.
Regarding your second point, as I've said before you are flat out wrong as far as the high-level jobs in Silicon Valley go, and I speak from inside experience. The market is competitive and foreigners get paid equivalent wages.
You are conflating low-level, relatively low-skill IT jobs with extremely high-level jobs at Silicon Valley.
There's a massive gap between the two.
