"What would you think about a law that said that blacks couldn’t get a job without the government’s permission, or women couldn’t get a job without the government’s permission, or gays or Christians or anyone else?" George Mason economist Bryan Caplan asks. It's a pretty easy question. Obviously, such a law is discriminatory on its face, serves no rational purpose, and is unacceptable in a liberal democracy. But Caplan continues: "So why, exactly, is it that people who are born on the wrong side of the border have to get government permission just to get a job?"
Well Bryan, it is because they don't. Someone born in Canada does not need the permission of the American government to get a job. They can take any job they want in Canada, or anywhere else in the world, for that matter, without checking with the American government. They can even take a job with an American company without getting permission from the American government, so long is that job does not involve moving to America.
It is not the "getting a job" part they need permission for, but the "moving to America" part. So once the situation is rightly understood, the question becomes, "Why should anyone not currently part of the polity of the United States of America need to get the permission of its government, the body recognized as holding sovereign power in that polity by the vast majority of its members, just to join that polity?"
But once you phrase the question sensibly, the answer becomes pretty obvious: a group has the right to control who can become a member of the group. If it loses such control, it will soon cease to be a coherent group. Since Caplan is an anarchist, this is probably what he wants: for the United States to fall apart as a coherent social entity. But it really won't do to be upfront about that, will it? "I favor open borders because it will lead to the collapse of the United States government" is not going to find as much favor as the pseudo-argument he actually uses.