No offense, but that's a pretty bullshit argument.
For one thing, the NFL players aren't protesting about a lack of freedom, they're ostensibly protesting the behavior of the police (although some seem to have taken that a step further into race issues in general). Ironically, they're making their point by protesting a symbol of the freedom that allows them to protest. Not a real strong start by Kap & crew in the logic department, but that's not really the point either.
Freedom of expression means that you are free to express your opinion without the government stopping you or coming after you. That's all. It does not mean others are compelled to lend you their own forum to express your beliefs, which is really what this is about.
If someone else put a Trump bumper sticker on my car, I'd be free to take it off. If I call up the newspaper and tell them they need to do a story on ... let's say, chemtrail conspiracy theories ... they can decide whether it's in their best interest to run the story or not. They don't have to. Hell, if I come to this forum constantly extolling my opinion that the Patriots are the best team in the history of pro sports, the site owners are free to tell me to shut up or GTFO. It's not infringing on my freedom; the government isn't suppressing me. Someone else just decided their audience doesn't want to hear it and I need to go find my own way to spread my message.
Same idea here. Players want to use the NFL's TV platform for their own personal views, and it's pretty well-established they're not guaranteed the right to do that. You put a decal on your helmet, or wear a headband with your message on it, they can tell you to take it off for violating the uniform policy. You go on America's Got Talent and launch into an anti-abortion tirade instead of singing or dancing, and they'll kick you off the stage.
Hell, if I decided to put a Black Lives Matter banner at the top of all the pages on our company website, and the owner said "I don't want that on the site, focus on promoting the products," and I argued with him about free speech and how important the issue is to me personally, guess who wins that argument? Now, the one thing I agree with 100% is that the NFL completely bungled the handling of that; I mean they have 30+ years of precedent in their own league of controlling very tightly what is an acceptable way of conveying your personal beliefs on the field (mostly: don't do it). But button-mashing Roger just sort of follows every media trend like a guy chasing a bus, so that's how we got here.
One final note - I find it really ironic that a lot of the people complaining about the players' right to express themselves on the job are the same liberals who have a well-deserved reputation for trying to get people fired from their jobs, kicked out of school, blacklisted from any prominent position, etc., for things they do on their own time. I mean, these are the same people who go around doxxing people over Twitter comments, calling people's employers demanding they be fired - real petty, below-the-belt shit like that. There is a fundamental lesson in that from which they would benefit, although they probably don't even see it.