"To believe that patriotism will not flourish if patriotic ceremonies are voluntary and spontaneous instead of a compulsory routine is to make an unflattering estimate of the appeal of our institutions to free minds. We can have intellectual individualism and the rich cultural diversities that we owe to exceptional minds only at the price of occasional eccentricity and abnormal attitudes. When they are so harmless to others or to the State as those we deal with here, the price is not too great. But freedom to differ is not limited to things that do not matter much. That would be a mere shadow of freedom. The test of its substance is the right to differ as to things that touch the heart of the existing order.
"If there is any fixed star in our constitutional constellation, it is that no official, high or petty, can prescribe what shall be orthodox in politics, nationalism, religion, or other matters of opinion or force citizens to confess by word or act their faith therein. If there are any circumstances which permit an exception, they do not now occur to us."
--Supreme Court of the United States, West Virginia State Board of Education v. Barnette, 319 U.S. 624 (1943).
You are missing a couple of main points here.
(1) People watching someone being openly disrespectful to a country that they love have EVERY RIGHT to show and express their disapproval and to decide that they no longer want to be a part of that event.
(everyone agrees right?)
(2) He did this while under the employment of the team owners who have every right to require a certain level of decorum and behavior while under their employment.
(3) When on there own time everyone has complete freedom of expression. Everyone agrees with that.