What possible reason could there be to ban off-duty police from carrying guns at NFL games?

AWR Hawkins notes that there are at least two stadiums where off-duty police will be able to carry guns (both in Texas), but why the NFL would want to ban off-duty police from carrying is beyond me.  Would the NFL ban on-duty police from carrying?  No.  So why don't they trust off-duty officers?  From Breitbart.com:

On October 7th, Breitbart News reported the NFL's ban on off-duty officers carrying their sidearms into NFL stadiums. On February 18th, Breitbart News reported the Minnesota Police and Peace Officers Association was suing the NFL, claiming the ban on off-duty officers carrying guns "violates state law." . . .