He has the right to exercise his views.
Now, if his contract or if some general NFL edict makes clear he can't do that, then he should be given a warning. If he does it again, then take whatever action is allowed by his contract.
He doesn't need to like or respect this country to be here. He doesn't need to be mature to play football.
It would be ideal if he was enjoying this countries society, football, and his teammates, but if he is not, he's not.
There are a lot worse things people do everyday in this country. He stands out because he is in the public eye. When you look at the big picture, what he did wasn't a "federal crime."
If you want to talk about people that are causing this country harm, look to Hillary Clinton, then we've got a real conversation going.