Right, even if one starts from an aggrieved position of "They took our jerbs!" after it's happened you do not want to let those workers be underpaid or abused, because it will (A) only increase the incentives of employers to import pliant immigrant labor and (B) it'll ripple out to harm the bargaining position of all "locals" still in the same line of work.

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.