There seem to be stricter rules for awards on ATA-operated flights (no unpublished routing option):

"For one Award, you can fly roundtrip on any service we sell in the continental U.S., which includes codeshare or ATA Airlines-only service to destinations like New York’s LaGuardia Airport (LGA), Washington D.C.’s Reagan National Airport (DCA), and Dallas/Fort Worth International Airport (DFW)."

Source: http://www.southwest.com/rapid_rewar...tions_faq.html