Since Superman is from Krypton and is eligible only to be a naturalized US citizen, but can never be a "natural born citizen" or become president of the US, and possessing super powers as he does, being a US citizen could be a bit stiffling. Maybe he wants to be a "citizen of the planet" or a "citizen of the universe." People who have a problem with it need to get over their egocentricity. That, and the fact that Superman is fiction anyway.

Sg