Yes, they can be fired.  But, it's left up to the organization.  Since our stupid President refuses to abide by his oath and his treasonous followers want to overturn the Constitution by stepping in to the private industry and punish citizens for exercising their right afforded by the Constitution, it's good to see someone have the kiwis to be Americans and shove it back in their faces.

You need to read the rule more carefully.  Standing for the anthem is a "should", and no mention of penalty associated with not doing it exists.  The penalties mentioned in that clause relate to being present for the anthem, and even then it's left up to the league what punishment, if any, will be issued.

EDIT:  While you're at it (reading), go read the Presidential Oath.  It's only a few sentences.  Its purpose is to lock the President into protecting the Constitution, which this idiot is doing the exact opposite and should be brought up on charges.