But they are in fact full-fledged human beings living physically in the United States. They are buying groceries, paying rent, getting their haircut.
Once you realize they are fully participating economic actors, it is much less clear whether they are a net positive or negative.
As if your (very American) bosses wouldn't find another to put you down and wrong everything they could from you. Today ICE is going after immigrants, tomorrow Pinkerton is going after you.
P.S.: Conversely (inversely?) there's often a vague idea of "if we somehow make the positions suck ass, then immigrants will stop coming, and somehow locals will be hired to a no-longer-sucky position", which is, er.... Wishful thinking. It'll get one a sense of vengeance, but it wont help pay the bills.