"... 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."
Wrote Justice Jackson in West Virginia State Board of Education v. Barnette, the Supreme Court case that that recognized a free-speech right of public schoolchildren to resist compulsion to recite the Pledge of Allegiance. That's quoted by Jeffrey Toobin in a New Yorker article about Colin Kaepernick's refusal to show the customary respect of standing during the National Anthem as it is played before professional football games.
Toobin acknowledges that Barnette is about a legal rights that can only be asserted against the government. He doesn't mention that compulsion to recite a pledge is very different from compulsion to show respect while somebody else is saying/singing words that are not even a pledge. And he doesn't mention that Kaepernick chose to join a team, while the children in Barnette were compelled to go to school. And children compelled to go to school are required to follow dictated behavior standards while they are taught material that may be designed to instill patriotism or other political dogma that they might not believe.
And, most importantly, Kaepernick is not being compelled to stand during the national anthem. He sits or takes the knee and we all get to see that. He's got his freedom of expression. He's just being criticized for what he is expressing. There was absolutely nothing in Barnette that saved the schoolchildren from being regarded as bad and unpatriotic for failing to say the pledge.
The criticism too is free speech.