At this point it's more tradition than anything. But if Metallica can perform their own interpretation of the anthem, then players can listen to the anthem however they want. Standing, sitting, I don't care. I would wager for many performers it's feigned patrioticism for more publicity. Hell, even half the audience doesn't really care about the anthem. Look around. People talking, getting concessions, etc. Meanwhile at international soccer events I see other countries belt their anthems with far more passion.
It's fine to have the anthem before the game as tradition, but it's overuse has undoubtedly marginalized its sanctity.
For all the fuss we give players sitting or taking a knee, I think we should note that they are still silent and respectful of those who do take the anthem seriously.