Markets work. Cutting off the vast supply of minimum wage international workers will incentivize Universities to treat and pay their junior researchers better.
As the parent insinuated, stipends are set by the NIH and congress, not the university. They are largely paid out with grant funding from the government. If an entity wanted to pay a researcher more, it can't come from federal funds which is how most labs (outside of big ones that have industry collaboration) primarily fund themselves. So while markets work, this isn't a competitive market, it is a regulated one where a ceiling is being set.
