One of the religious justifications offered by those who support illegal immigration is their contention that the bible contains multiple exhortations to "welcome the stranger". Religious leaders transmogrify that into a command to support people moving to the U.S. at will. For the religion-based answer, see this and this. There isn't a non-religion based answer that I know of, but those who make this argument can be discredited in other ways based on simple morality. See the false compassion discussion.