Superman was an illegal alien working under an alias for the mainstream media.
Lex was trying to FREE you people!!!
Superman was a foundling, and as such would be considered a U.S. citizen. If he weren't a fictional character of course.
Per federal law, 8 U.S.C Sec. 1401 grants automatic citizenship to: "a person of unknown parentage found in the United States while under the age of five years, until shown, prior to his attaining the age of twenty-one years, not to have been born in the United States"