As far as I know:
1. Kaepernick wasn't fired. He simply wasn't signed by anyone team after his contract with the 49ers ended.
2. It's a matter of factual controversy whether his treatment by the NFL was affected by his advocacy at all. As far as I know, no manager has explicitly admitted to making different choices about how to deal with him based on his kneeling.
3. It was never suggested by anybody that Kaepernick's kneeling might be a breach of his contract.
4. Kaepernick was not denied his pay for matches he'd already played in as a consequence of his kneeling.
Assuming I am correct on the facts, there is, at the very least, a significant difference in degree between that case and this one. Do you claim that anything I say above is wrong?
It also seems relevant here that basically all coverage I saw of Kaepernick's case - from the nearly-exclusively right-wing commentators I follow - was harshly critical of the minority on the right who were calling for him to be punished. By contrast, I have never seen anyone on the left criticise speech codes or corporate censorship. I do not think it is reasonable to try to draw an equivalence between the right and left on these issues by comparing the positions of a minority on the right, heavily criticised by other right-wingers, with the position of an unchallenged hegemony on the left. There is a real asymmetry here, both in terms of what the majority position of each coalition is and the extent to which they actually punish the speech they disfavour in practice.