The whole National Anthem things is a black civil rights thing. This goes back to the fist in the air in the 1968 Olympics.
I can't think of many other causes from any other fringe, disgruntled group that used the Anthem as a protest like Black Americans. Today it is completely attached to (in my opinion) to the Black Lives Matter Protest.
If you are condoning this behavior, you are really giving approval to the Black Lives Matter ideas. I mean, the same people that say these players have the "right to kneel" would be in utter outrage if a white player stayed in the locker room, raised a right arm and played the Third Reich National Anthem and stood in front of the Nazi Flag, then refused to stand during the National Anthem. What if a player wanted to where a confederate flag on their shoes as a sign of support for the keeping of the statues up? What if a player wanted to where a "Make America Great Again" hat on the sidelines instead of his team hat?
When the NFL allowed this in the very beginning, it was a sign of support for the cause. It was a sign the owners took a position and sided with the opinions of Black Lives Matter. Period.
The hypocrisy to think otherwise is just mind numbing. This is big city, black America, democrat voting owners deciding their position is more correct than the opinion of Middle, white America, Republicans. Whatever side of the Black Lives Matter debate you believe in, you can't deny this is what is happening. The NFL has chosen it's position and become politically left, whether intentional or not. For their fans in democratic demographic areas (big cities, minorities), this is fine. For their fans in rural, middle America, republican dominated demographic areas, not so much.
The NFL decided to pick sides long ago with their action/inaction.
Trump is just riling up his base because he sees that the NFL has become liberal elitists and they are an easy target. It's how he get voters to turn out.