He is using (abusing) his employer's platform to express his personal disrespect for the anthem of the nation that has brought him wealth and fame, a disrespect which is not shared by a good portion of his employer's customers and hence a direct slap in the face to them, those who pay money to make him rich. He causes upset and divisiveness in an arena that should be for entertainment and relaxation.
Standing for the anthem doesn't mean you have to agree with everything the U.S. has ever done or is currently doing. But not standing for it demonstrates contempt for that which it symbolizes: A nation born out of the desire for Liberty. I'll have no truck with that; if you hate this country that much, get out.
The Rights enumerated in the Constitution are the black man's best hope. The Anthem celebrates a few white men breaking from the tyranny of a king, it does not celebrate the fact that some of them owned slaves. It took another century to see the light with respect to race which was more about recognizing that ethnic Africans were in fact fully human than about there being anything wrong with the Constitution or the concept of individual liberty.
So if you cannot grasp all this or at the very least fake the body motions of an opening ceremony (just stand, cross your fingers behind your back if you must) to an event benefiting you financially enough to keep you richer than 99% of your white "oppressors" then yes, what you are doing is wrong. I may agree with your opinions about too many blacks being incarcerated but this method of protest is counterproductive. Use your own money to buy your own billboards. Keep it out of the games.