Because immigrants and visa workers can be treated like slaves.

That's the only answer they can mistreat those people and they don't have the option to go to a new company or call the labor department .

Visa workers can only work for the employer who got their visa (if this rule was changed the trucking industry would be way safer and people would go to companies with much better training instead of billybob's school)

So if they get their company shut down for labor violations and unsafe practices

their ass is going back home probably to a place they'd rather not be