Taking a knee is NOT their symbol. It has been symbolic of surrender and servitude for far longer than Black people have lived in the US and for far longer than the US has been a country. Maybe Black America should stop appropriating other cultures' symbolism. The traditional pose for christian prayer is to get on your knees thereby symbolizing one's submission to god. Muslims completely prostrate themselves in submission to allah.
When one is knighted, one takes a knee demonstrating their servitude to the King or Queen. Even in football it's an act of surrender, so to speak.
But hey, if Black people want to demonstrate their submission or subservience to the US, it IS a free country after all.