Actually, it's not their 1st Amendment Right (I know that's going around as justification, so let me explain). They're paid employees who are subject to the rules of their employment contract. If their actions are in violation of their contract, or if they bring a negative light to their employer, then they can be fined or fired. Under the logic it's simply them exercising their rights, you could apply that same logic to any type of offensive speech at any employment. So where does that leave us? The NFL is condoning and supporting this expression of speech. They have not supported teams and players when they wanted to show support for such groups and causes such as 9/11, the Dallas PD, and Breast Cancer. So this is chosen support for a specific cause demonstrated physically. I have also heard the excuse that it's unique because it's simple physical expression rather than verbal or uniform modifications. If you believe that, just ask yourself how the NFL would respond if one of the players used the Nazi salute. Yep, that would be disgusting on a whole new level, but the form of expression and protest would be the same. The NFL should stick to their business.
Ash, I'm offended by it... but I agree in general with you as I've fought to defend people's rights. If they want to express themselves this way, that's their choice, i don't have a right to a world where I'm not offended, and no legal consequences will or should follow. The question is, are they willing to live with the consequences to their employment and their public reputation by expressing themselves this way? For most of them my guess would be... no.