Comment by JuniperMesos

But foreign PhD students and postdocs who are being paid partially in the legal right to reside in the US might well be willing to accept those conditions. Just as an H1B visa tech employee is willing to accept lower wages and less freedom to challenge their employer, or an illegal immigrant farm laborer is willing to accept those working conditions in return for not being in whatever country they illegally immigrated from.

Any justification at all for the US government to give a visa to someone - including student visas, including visas for postdocs doing ostensible research - will be gamed by people whose primary concern is access to the US. Demand for access to the US among the myriad peoples of the world is that strong.