Noone has even suggested that Russell is a US citizen. Of he had been obviously there wouldn't have been any problem in him overstaying on his visa? US citizens don't yet need visas to live in their own country.

He's an Australian with an American daughter, who isn't allowed to live in Australia because of her disabiity and the Australian authorities, and he can't come to join her in America because of the American authorities.

Comprendez?