Unless they aren't eating or sleeping while they're in the US, they're still spending some of their money on US businesses. And remittances are by far the most effective form of foreign aid...

Do they not purchase clothing? Food? Housing? Leisure items? Vehicles? Electronic devices? Basic necessities? That are designed and produced by Americans, sold by Americans in American stores in America?

As for remittance, I believe that people are free to spend their disposable income however they wish.