What makes America different from North Korea or Communist China or Thailand? In all three countries, criticism of the government is forbidden, and worship of the symbols of state is mandatory.
In theory, the USA is a country of free people; people who are free to stand, salute, sing, as the government asks – or not, as each chooses.
The day such obeisance becomes mandatory, the USA might as well quit pretending to be a free country. More than a few veterans have said much the same thing; they fought for freedom, not for mandatory flag worship.
I’m sure the NFL players would rather just play ball, but they’re not being given that option; politics has been interjected into the game already, and they must decide whether to pretend to follow the forms, or to protest. If we truly value free speech, then this is free speech. It should be allowable. If we say “not now, not then, not here, not there,” we might as well just say “shut up and stand on command.”