I'm actually surprised the league let him get away with that so long in the first place.
Not that I think wearing the upside down headband is a big deal, but I have no problem with the league telling him to wear it right. It's a voluntary part of the uniform.
Like I wouldn't expect to see a police officer wearing his hat backward, or the cashier at McDonald's wearing his hat/visor backward or visor upside down. But if I did, I wouldn't think badly of the boss who told him to wear it the right way.