It is an open gesture of disrespect to go down on one knee when any national anthem is played, in the US people are free to express that feeling without being dragged into a dungeon and that is what is happening. One of the things that make America great, you can spit on it and live to tell the tale with a high level of media adherence and sympathy even. Sections of it.
It is hard to envisage any nation's President endorsing such acts, it means those people are ashamed of being citizens of that country in no uncertain terms. He has his own style but I totally agree with the principle he upholds.
Many black players have enough respect for the anthem to stand on both feet so you don't have to be white to agree with the President. These practices are socially counter-productive, they cause more division that unity. In that dressing-room, there is a split already.