Imagine if there was a law that required companies to post job openings for every position in the company, regardless of whether or not that position was already filled. As you can imagine, they would likely make the job requirements exceedingly specific for the roles filled by people they really like.

You don't even need to imagine, just go look at government or non-profit job postings. It's often the case that they similar ridiculous policies for promotions, where they already have a person they want to promote, but they need to open it up to public applicants first.

This is fairly obvious, but do Americans not realize this? Or are they consciously saying that its better for high paying jobs to move out of US than for foreigners to work at those jobs in US?

Nope, the alternative is off shoring the project to places like Ohio and Montana.

Being unable to import cheap labor from overseas would be a huge loss to these chop shops and allow other local companies to compete with local staff.