The mechanism is if there's one perfect candidate of the right age, race and caste but the person isn't US-legal, then companies will be forced to consider people of the wrong age, race or caste. Maybe not right away, but when the supply of the perfect age/race/caste US-legal people run out.
In my experience, the visas usually accentuate the distorted racial makeup of tech companies because they all go to overrepresented countries.
It's a fair point but AMA hasn't meant AMA for a long time, if it ever did.
I'm pretty sure that people would bring in about the same number of generic complaints / tangents as they usually do, regardless of whether AMA is in the title, so it seems like a red herring.