The big difference is that Colin did it on company time, and there’s no place of employment other than sports where a person can repeatedly disrupt business functions by interrupting everyone and then kneeling while someone is speaking.
Does kneeling during the anthem disrupt business functions? The game hasn't yet started during the anthem. Whether he stood or kneeled had no effect on the game, did it? It caused some people to talk which was the point.
A better question would be, if his team had fired him for his conduct, would you have supported that? Or would that have been a suppression of his free speech? What if he had done the same thing in support of Trump or some conservative movement? Would you be concerned about the deplatforming of a conservative voice?