Of course the players have a first amendment right to express themselves in any way that they choose.  But words and actions have consequences, where employers have a right to regulate conduct in the work place.  And the ultimate consumers have a right to express their dissatisfaction with the players' actions as well, by turning off the tv, stop attending the games and stop  buying the merchandise.  We'll see how long the owners put up with this form of expression once their cash cow begins to dry up.  

I also noticed that this past weekend in London, several NFL players knelt during the national anthem, but stood during "God Save the Queen" (England's national anthem), despite the fact that colonial England was hugely responsible for bringing many of those players' ancestors to the new world in chains.