Whether a team member should assume any particular posture during the national anthem before a sporting event should depend entirely on where he or she is.
Virtually all professional and college sports venues are built and paid for by the public. Until professional team owners build and maintain the stadiums they use, they remain public spaces. The Supreme Court has been clear that “the public square” is a guaranteed venue for free speech with limitations confined to safety. Except for an event in a privately-owned stadium, that leaves the posture decision to the individual.
Sports venues are by their sheer size and scope, some of the most public spaces defined by unemotional logic. Why this is even a controversy is defined solely by the emotion animating our current public debate of, seemingly, all things.