The American electorate believes unemployment is caused by an improper ratio of people to jobs, and it can be solved both by increasing the number of jobs and decreasing the number of people.
In this theory, your friend's expulsion was a victory for the American who took over her job, and for the unemployed person who entered the labor market (if not directly, then via a domino effect).
We are comparatively loose with immigration statuses that don't permit work. Tourist, student, etc. visas are relatively trivial to obtain. We have programs that allow work which doesn't compete with Americans, such as international performing arts groups on tour. We even have programs to fill jobs that Americans aren't available to fill (H1B for highly skilled labor, seasonal farm workers, etc).
The moat is built around statuses that permit competition with Americans for jobs in general. It remains this way because Americans do not generally consider or believe Econ 101 ideas like that trade necessarily benefits both sides of the exchange (or they would not make it), more workers also means more consumers, cheaper labor => cheaper goods and services => higher standard of living, etc.