I did not at any moment say ti was acceptable, I said it was LEGAL, In fact, I think it should be illegal. You cannot fire people because they are women, because they are old, or because if their race if the person can prove it. The reality is that when a person is fired he STAYS fired. If he wins a case, as a rule, the company pays some sort of compensation, which is almost always confidential.
They can certainly fire this football player for his refusal to stand. But I don't think they will. It would make the owners look like dicks (which they would be) and many fans would stay home.
As I said, there is an article in the Nation about his position, which clarifies it a lot.