What you said was that they don't ask for it. They might not, but they are entitled to it. And at least here they are not reported as illegal because the state cares more about them getting compensated than their legality. Plenty of them DO get compensation, especially if they work on farms (here obviously, I'm not commenting on other states where I don't practice).
And I agree with you, too, because they do do jobs that Americans don't want to do and are poorly-compensated for that work, which is often dangerous.