If there are n tech jobs to be done, y people in group A who can do them, and z people in group B who can do them, opening hiring to all (y + z) people makes hiring easier than confining yourself to only y.

Easier could mean easier to hire, cheaper to hire (even if you pay them all the same as each other, the clearing wage is likely lower due to increased supply), easier to retain, etc.

Because they can serve them shit-sandwiches in other metrics and the H1B has no recourse.

"Work over the weekend or your employment will be terminated and you will have to be deported."

probably because of non-pay related practices? things like differences in healthcare laws, unionization, legal rights as a citizen (of a particular country) vs being a visitor who can be evicted/deported at will.

Because when a minimum wage is fixed, the market naturally simply adjusts the other parts accordingly.

Sure you pay both $x, but the skill diff can be 10x. USD16$250k can get you a bootcamp coder w zero exp (but w profile/status of citizen cardholder) [1] or someone way beyond 10x that skill but who simply can't afford to negotiate.

[1]: https://www.businessinsider.com/haseeb-qureshi-how-to-negoit...