When we go to a movie, we pay for a performance rather than purchase the actors. Similarly, when we go to concerts, we do not control the musicians or the music. Regarding athletes, is there any difference? Even if they are paid to endorse products, they are not for sale.
Our superstars are pleasing to us, even enough to sell things, so we try to make heroes of them and then expect them to behave that way. Traditionally, heroes showcase character and virtue where superstars give us glamor and talent. These categories do not often cross-over even though both may be celebrities. Different aspects are offered to be judged.
I am neither an athlete nor a sports fan, but I am interested in the debate if football players refuse to give a conventional salute to our flag. It seems to me that players put their heart in the game at personal risk giving a best effort to win. It is the game that counts. We may prefer they behave in certain ways as non-players, but they do not owe us anything despite high pay.
Like the rest of us, they may wish to stand for what they believe not what they are told to believe. We can criticize them, but it might be helpful if we tried to understand what they attempt to communicate. Rather than an offending patriotism, for example, they may be asserting a belief that America is not living up to its standards or promise for justice and equality. This implies a belief that America could do better if it sees the problem better. So, they use their fame. Looks like a question of honor to me rather than an insult. They certainly will not gain material rewards for this stance.
If fans insist that their own standards prevail, then that requirement should be spelled out in the contract. When I worked for state government, I was not permitted to publicly support candidates, which means I knew the rules in advance that would curb my First Amendment rights. If the leagues want to enforce a sort of loyalty oath (although these players are criticized for something they do NOT do) making employment contingent on agreement to take no political stands at games, then players may decide not to play on such a team or in the game at all. The game then loses the great talents they paid to see in a great performance in the first place. We are all free agents in this sense – or should be.