That is irrelevant. They get paid more compared to what immigrants would get in their home countries. And if you had to pay them as much then they would lose the edge during hiring - it is part of why they are also chosen.
However wage arbitrage is specifically against the intent and the letter of visa programs. It’s to fill actual shortages. If you can’t find a Python dev at $70k per year but you could find one at $100k per year, that isn’t a “shortage” — yet such “shortages” are what gets used to rationalize H1-B expansion.
I'd like to add that part of the factor that's driving down wages is the fact that workers from countries with cheap/subsidized higher-level education can get a bachelor's degree for under $5000 (total), meaning that a student from a foreign country getting a master's degree from an American university graduates with around 1/3 the total debt that an American student would. That advantage makes it more feasible to take lower paying jobs in the first place, thus broadening the talent pool, which inherently drives wages down.
Thats not how salaries are negotiated. Any worker picks the job that suits him best. If you have A and B in the US, and C in your home country, you will pick the best of the three, not "not C".
