Yeah, it's the usual bullshit excuse. Immigrant workers, because of their status as immigrants, are willing to endure a certain amount of "exploitation" (in multiple forms - be it long hours, not questioning superiors, lower wages, etc.). Hence, corporations (in our capitalist system) and businesses of all sizes (big and small) prefer to hire immigrant laborers/workers over the local population.

Note: This phenomenon is NOT unique to the US. This is how Capitalism works. This happens in ALL countries - US, Canada, UK, EU, India, Russia, China, Australia, Middle-East countries, etc. Capitalism goes where the labor is cheapest (in an effort to increase profits regardless of the damage to societies, cultures, ecosystem, or anything else).