Residents should return the forms but not visitors. Do tourists get to submit a census form if they frequent Florida? Maine? California???
Illegal immigrants hold no residency in the United States. Fox wanted them to be able to continue to vote in Mexico. Ergo they hold no political sway in America and should not be used as justification to steal Congressional seats from legal residents.
A more relevant question is do FOREIGN tourists fill out census forms when they visit the US??