From What We Owe Immigrants, a blog post by Bryan Caplan:
Suppose two men, John and Julio, are heading to a job interview. Julio tells John: “I need this job more than you do. Please drop out of the race so I get it.” It’s perfectly reasonable for John to make Hardenberg’s reply: “No. You’re a stranger and I don’t owe you anything.” At this point, Mangan and I are in full agreement.
But suppose instead that John handcuffs Julio to a tree to prevent him from going to the interview. Julio says “Let me go. I deserve a shot at this job too.” At this point, it’s ludicrous for John to reply, “No. You’re a stranger and I don’t owe you anything.” Julio isn’t demanding help; he’s just demanding that John leave him alone. And if John were to object, “You’re not leaving me alone. That job is MINE, and you’re trying to steal it from me!” we’d have to answer, “The job isn’t yours. It’s up to the owner of the business to decide who he wants to employ.”
All of this is obvious to any upright 10-year-old. You’re under no obligation to give your toys away to less fortunate kids, but you’re certainly not allowed to steal toys from less fortunate kids.
Unfortunately, if the victims happen to be born in another country, most adults don’t have the moral sense of a 10-year-old. Don’t want to help poor foreigners? Fine. But at least leave them free to sell their labor to willing employers, rent apartments from willing landlords, and buy goods from willing merchants.