I believe what the football players did falls under their first amendment rights of expression. Where I think they are wrong was they did not notify their coach according to sources that they were going to do what they did. Also football is a team sport and I wonder if they have the support of the rest of their teammates or if those on the team who did not partipate in the demonstration did not support what they did.

No matter, it is their right to express themselves as long as that expression violates no laws or harms anyone.