I wouldn't think the NFL does, which is why it's weird that the letter is coming from them. When vendors outside Wrigley were selling the "Horry Kow" merchandise, it was the Cubs that made them stop, not MLB. Not exactly the same thing, but similiar I suppose....
the league is really reaching on this. The phrase originated in the late 1800's. Just because it's been applied to the Saints doesn't mean the NFL should be able to control it. The only way they should have any say is if Saints logos are used.