Correct, kneeling during the anthem is not something that would be protected under the 1st Amendment. But if you paid attention to the NFL this weekend, it's pretty clear that the teams and owners are not setting rules about this and therefore the players are not violating any such rule. In fact, it looked to me like most of the teams and owners openly support this protest by their players, some of them even joined in.

And at any rate, the fact that it's not legally protected under the 1st amendment doesn't change the fact that it's still an expression of the right to protest. Our country was founded by protesters of their government, and free protest of the government was expressly written in as a core belief of our nation. Seems that the owners recognize this and are not limiting their employees in this case. So the people who have an issue here are the ones who need to reconcile how their personal reactions to this protest conflict with the core values of our nation.