If standing for that national anthem is not political, why is it done? Just because something is not controversial for any of the paying customers does not make it not political. For crying out loud, it is claiming allegiance to our country which is, by definition, a political entity. You can't possibly claim that it is not inherently political unless there is some other utility. Does it help the performance of the athletes? Does it give extra time to coaches to game plan? For the refs to huddle and make sure they are up on rules?
I do agree that employer generally gets to decide on player displays of political activity, but I also think that employers generally don't require displays of political allegiance. If my employer required me to sing to the national anthem at the beginning of my work day, I damn sure would think of that as a political statement. Hell, if it's not political and paying customers agree with it, why is this not generally accepted practice in every industry?