Many situations exist where the final outcome is that a US citizen loses their job to someone who is not a US citizen. In every case it appears that loopholes are involved, where the stated purpose of the law or the regulation is undercut by lack of definitions of terms, lack of enforcement, or lack of reporting or any number of other loopholes.
As long as it is highly profitable for firms to employ non-US citizens, they will continue to do so. Thus, federal job stimulus money boosts the economies of India and China more than it does any other. When the stimulus program ends, both China and India see a dip in job growth.
Close the loopholes, eliminate the pathways from L1 or H1b visas to green cards to citizenship, and make immigrants wait their turn, as our forebearers did.