Actually it is. They are paid to be there. Management expects them to stand up. If you want to protest do it on your own time. As Colin has learned that you cannot defy your employer and then no one else wants to deal with you.
In actuality I agree with him but you don't spit in the boss's face and expect no reaction.
Perhaps Colin and others need to pull a Hobby Lobby, and assert that the NFL is violating their religious freedom.
After all, a person's religion may prevent them from ritualistically worshipping a false idol, and false idols can be determined by the religious practitioner. It may turn out that the flag would be considered a false idol until the justice it represents is actually delivered.