That is, when we talk about immigration controls, we are not simply referring to a sign at the border that says: "Do not enter without the official permission of the U.S. government." No immigrant would pay any attention to such a sign.
Thus, to enforce the immigration-control system, the federal government increasingly enacts immigration-enforcement measures, which inevitably involve a violation of the libertarian non-aggression principle.
They stop people crossing the border and conduct warrantless searches of their bodies (including sometimes body cavities), cars, and cell-phones. Everyone has become so accustomed to this because it's the system in which we have all been born and raised. But it truly is a shocking notion that a people who claim to be free have to subject themselves to this type of thing just because they are returning from a foreign country.