They have the ability to do it because they are not "operated" by a govt. entity. The key word in 9.41.300 is operated.
(2) Cities, towns, counties, and other municipalities may enact laws and ordinances:
(b) Restricting the possession of firearms in any stadium or convention center, operated by a city, town, county, or other municipality......
If qwest field was operated by a municipality then there is an exemption for people with a CPL.