There is no such duty to hire non-foreign-nationals. I do however have duty to my company to make the best hiring decisions, so I'm not going to pass on a perfectly qualified candidate with an H1B and cross my fingers that I'll find a similarly qualified candidate who also happens to be a citizen soon. The opportunity cost can be quite dramatic. We pay H1Bs the same as anyone else, so technically they end up costing more - sometimes this offsets said opportunity cost, often it does not.
I also don't hire for "American" jobs. They are just jobs and if you can do it, I don't care about your race, ethnicity, origin, preference of Star Trek to Star Wars, etc. On a macro level, I believe there's more value to the US with the foreign nationals bringing their labor and keeping its products here rather than competing with US companies from overseas. Let's not forget that these foreign nationals pay taxes and spend money in the US as well.
I also don't hire for "American" jobs. They are just jobs and if you can do it, I don't care about your race, ethnicity, origin, preference of Star Trek to Star Wars, etc. On a macro level, I believe there's more value to the US with the foreign nationals bringing their labor and keeping its products here rather than competing with US companies from overseas. Let's not forget that these foreign nationals pay taxes and spend money in the US as well.