Kneeling.. Is this a First Amendment Issue?

Colin Kaepernick, along with many other athletes, have decided to kneel instead of stand for the National Anthem. Many say it is his "first amendment right" to not have to stand for the National Anthem. However, this is not true. Only the government can violate someone's first amendment rights, not a privately owned company. The NFL has the right to not renew his contract for kneeling during the National Anthem. Colin Kaepernick would not have the grounds to sue them for violating his first amendment rights. He could potentially sue for other things but not them violating his first amendment rights.

My personal opinion of the issue is that it is extremely disrespectful of Kaepernick, and every other athlete that refuses to stand during the National Anthem. There are people that die for them to just be able to play football and they do not even have the respect to stand during the Anthem? It is extremely disrespectful and ungrateful.