Why do players take a knee?

[Not Ruth]

4. I thought that it was a form of self defense, i.e. non violent resistance to police brutality

You are not allowed to be attacked.

"The 2011 NFL Rules state in Rule 7, Section 2, Article 1(c): "An official shall declare the ball dead and the down ended … when a quarterback immediately drops to his knee (or simulates dropping to his knee) behind the line of scrimmage".

The 2011 and 2012 NCAA Rules state in Rule 4, Section 1, Article 3(o): "A live ball becomes dead and an official shall sound his whistle or declare it dead … When a ball carrier simulates placing his knee on the ground." The same rule is used by the British American Football Association.

The 2011 CFL Rules state in Rule 1, Section 4: "The ball is dead … When the quarterback, in possession of the ball, intentionally kneels on the ground during the last three minutes of a half". The Statistical Scoring Rules, Section 5(e) states: "When a quarterback voluntarily drops to one knee and concedes yards in an effort to run out the clock, the yards lost will be charged under Team Losses. NOTE – No quarterback sack will be given in this situation.""