In a nutshell:
When he's in his uniform, on TV, during the opening for a scheduled game, he's representing more than himself. He represents the team, the conference, the league and to the degree they support him financially, his fans. He also represents his sponsors.
He has the right to protest however he thinks is going to make a difference. That in no way means his choice is without consequences -- not for stating his opinion, but for the place, time and manner in which he chose to do it.
Remember Congressman Joe Wilson? He interrupted Obama's speech with "You lie!", and was rebuked formally by the House of Representatives. He wound up apologizing:
"This evening I let my emotions get the best of me when listening to the President's remarks regarding the coverage of illegal immigrants in the health care bill.
While I disagree with the President's statement, my comments were inappropriate and regrettable. I extend sincere apologies to the President for this lack of civility."