Standing for the national anthem is not a politically neutral action. FORCING players to stand for the national anthem is not a politically neutral action.
It's common, but that doesn't mean it's neutral.
Oh brother ... but OK.
The NBA went through something like this. Their policies called for standing for the anthem if you're out on the court while the anthem is playing (i.e. and cameras are rolling, and fans are watching). If you happen to have an objection to this, then you are welcome to stay in the locker room for the duration of the anthem and come out after.
I think that's entirely fair. You don't get to politicize a neutral platform, and you also aren't forced to do anything you morally object to.