Nope, NFL players have every right to protest the flag, or anything they feel is wrong, as long as the team owner allows it (because, yes, an NFL team is a business and the owner does have the right to say what happens within his buisiness).
Other than that, players can protest all they want, and the people can react with their wallets. It's the very thing that America stands for. Ironically, the very thing that our men and women fought and died for is what gives them the right to protest. Take that away and they all dies in vain.
So, let them protest, and you, in turn, can protest by not buying NFL tickets or merchandise.