Part of the case for open borders is that at the very minimum, our obligation to strangers is to leave them alone. Immigration restrictions, by denying strangers the freedom to move to another country, violates this basic obligation.

It is conceivable that individuals have no positive obligations toward the welfare of those from other countries. Thus, there may be a case that immigrants (or for that matter even natives) do not have a right to one’s charity. However, preventing people from immigrating is a form of active harm rather than simply failing to provide charity. See also the killing versus letting die distinction.

Hypotheticals

• Starving Marvin, a hypothetical by Michael Huemer that illustrates that immigration restrictions are akin to forcibly preventing a hungry person from going to the market rather than refusing him charity.
• John and Julio, a hypothetical by Bryan Caplan that illustrates that immigration restrictions are more akin to forcibly preventing a potential competitor from appearing at a job interview in order to increase one’s own chances of getting a job.

Arguments by analogy

Excerpts from The Stranger, a blog post by Bryan Caplan:

What are you morally required to do for a stranger? Not much. Even if he seems hungry and asks you for food, you’re probably within your rights to refuse. If you’ve ever been in a large city, you’ve refused to help the homeless on more than one occasion. And even if you think you broke your moral obligation to give, your moral obligation wasn’t strong enough to let the beggar justifiably mug you.

[…]

This would be bad enough if modern governments focused on forcing rich strangers to give to poor strangers. But it’s outrageous when the direction of coercion reverses. The most egregious example, of course, is restrictions on immigration. People in the Third World are strangers, but we still have a moral obligation to leave them in peace. Instead, we pass draconian laws forbidding these strangers to work for other complete strangers. And for what? To fulfill our fantastical obligation to maintain the wages of fellow citizens we don’t trust enough to give our kids a ride.