I'm pretty sure that doesn't make any sense. How, exactly, will allowing foreign nationals to move to the United States and seek employment cause harm?
Ultimately, these arguments usually come down to something like this: if people from less developed nations move into the United States and seek employment, the increased supply of labor will reduce the average cost of labor within the country and increase the burden on our public services. This is bad for the people who already live in the United States, so those people should stay in their less-developed nation, where they will have a lower quality of life but they won't reduce our own quality of life.
In other words, it's nationalist bullshit that places greater importance on the quality of life for U.S. citizens, simply because they were born in this nation and those other people weren't born in this nation. We're willing to let people outside of our borders starve to death, as long as it means that we won't suffer even the most marginal decline in our own quality of life. It's selfish, and the entire process of thought relies on an "us vs. them" mentality which places a lower value on the life of someone who lives outside the arbitrary borders of this country. Ultimately, allowing people to move more freely between countries will foster a greater emphasis on the importance of global welfare, instead of taking this "us vs. them" mentality that places the utmost importance on the welfare of our own citizens and is indifferent to the suffering of the global population.
I might add that a huge portion of this country used to arbitrarily belong to Mexico, but we conquered that territory in the Mexican-American War, taking over huge sections of territory that weren't even part of the initial dispute over the exact location of the Texan border. Now, in the modern era, immigrants traveling to the United States from Mexico is a huge cause for concern in the U.S., with people concerned that they're going to "take our jerbs", when in reality, those people are just trying to migrate into territory that originally belonged to their nation in the first place, before we took it by force of arms.
Furthermore, all of this anxiety that immigrants are going to ruin our economy is essentially unfounded in the first place, and is repeated ad nauseam by people with little understanding of economics who are making policy arguments that are based on ideologies that have been spoon-fed to them, about issues that they don't know anything about. The National Economic Council, the Domestic Policy Council, the President's Council of Economic Advisers, and the Office of Management and Budget published a joint report in 2013 which explains why immigration reform will ultimately strengthen the economy.