The right to protest is guaranteed in the US Constitution. The protesters are not hurting anyone. They are not causing problems for anyone. They are protesting the actions against a minority. This "They are stomping on the flag!" and "They are pissing on the graves of every soldier who ever served" is just bullshit.
How many freaking times does this have to explained??? The right to protest while on the job is not guaranteed in the US Constitution. You won't get arrested, but you might get fired.
You might. It will depend on whether or not your contract says it explicitly or whether you were failing to perform your job while you were protesting. Neither of those apply here.
That's exactly the point. Depends on the player contracts and the NFL rules. In other words, the Constitution doesn't apply here.