If people don't respect the emblems of their country, they have no right to participate in its public ceremonies.
Hijacking public events to declare one's political views smacks of arrogance. 'Racial injustice' is not the only issue in America. What if everybody decided to use such occasions to force their pet grievances on the notice of others? We couldn't go to a play, or visit the cinema or even enter a public park without being harangued: Save the Whale, Keep Atheism out of our Schools, Build more Railways, Build more Roads, Bomb Iran, Make peace with Iran.....
The social contract whereby we keep such declarations for the appropriate occasion would be broken, and with it a necessary restraint which makes social living tolerable.