There's little the NFL loves more than squeezing its players; they do it with celebration penalties, and fugazi league interviews, and — most pedantically — with the uniform code.
You can't do anything to your jersey or your helmet or your cleats, if you're an NFL player, or else you'll be fined. It's become a hard-and-fast rule that the league enforces at every opportunity. Once they stopped Peyton Manning from wearing black high-tops to honor Johnny Unitas, the horse left the stable.
Whether they meant to or not, the NFL painted itself into a corner.