They allow it if your cause is ‘acceptable’ to them.
Via Mediaite:
The Dallas Cowboys have been denied permission by the NFL to wear a helmet decal honoring the Dallas Police Department and the city as a whole — in the wake of the July ambush on officers in downtown Dallas.
The Dallas Morning News reported Wednesday that the league permitted the decals in training camp and practice, but rejected the Cowboys’ request to wear them during preseason and regular season games.
“Everyone has to be uniform with the league and the other 31 teams,” said Cowboys executive vice president Stephen Jones, son of owner Jerry Jones. “We respect their decision.”