The NFL fining players for wearing custom cleats or gloves that don't match their team's uniform is ridiculous, but it's in the rulebook and it's enforced. Most recently, it was enforced on Saints running back Alvin Kamar for wearing green and red cleats for the Christmas Day game against the Vikings.
The game was a historic one as Kamara tied the NFL record for rushing touchdowns in a game with 6 while rocking the holiday-themed cleats.
Kamara expected the fine to come and the money he's paying will be put towards a wonderful cause.
(H/T: Tom Pelissero)