I don't want you to miss the point, though, that if what I wrote above is wrong, that's good for the U.S., not bad.
When Jose tells Wal-Mart, "I'll do an hour's worth of work for you in return for some lemonade," that seems like a fair trade. Now suppose they do the deal and Jose tells Wal-Mart, "Okay, I did the hour's worth of work, but you know what? I'm not really thirsty. You can keep the lemonade. Consider my work to be a freebie." That's an even better deal for Wal-Mart.
It's just as true when you substitute the United States in for Wal-Mart. From our perspective, Jose's labor is a benefit, and the lemonade we have to give him for it is the cost of getting that benefit. If Jose does some work in the U.S. and then gets his lemonade in Mexico, that's awesome for the U.S. (and terrible for Mexico)!!!
That's not what really happens because Jose in fact gives his lemonade-voucher to Jim, who cashes it in for U.S. lemonade.
But if economics were wrong and Jose were really able to make his dollars disappear to Mexico, never to be spent in the U.S., that would be to the advantage of the U.S. To worry about that happening is to put the lemonade transaction in the wrong column in our cost-benefit table.