Why? Because of this:
Gov. Mark Sanford said today that he will not comply with the federal Department of Homeland Security’s standards for state-issued driver’s licenses and IDs, meaning S.C. residents could be subjected to extra security screenings when boarding airplanes or entering federal buildings.
Using a passport rather than an SC Driver’s License will (hopefully) solve this problem.
Update: Sanford’s objections to compliance are articulated here.