The man expressed his opinion. He did not use the force of law or send troops to enforce his view. I agree that if he had done that, that would be wrong. But the situation we have is a) he didn't and b) you're pretending he did to silence him.
It was an act of speech, not government.
If you can't handle free speech maybe you need to leave America. We built our country on it.
Those of us who are not Nazis.
Coercion & force aren't necessarily the same thing. The point of "take off the uniform" is to forfeit the ability to use the law or government office to ensure compliance.
Sending in the troops/police isn't the only means to use legal coercion, it's just the least subtle. If the President chooses to he can use various regulatory agencies, whom he appoints the heads of, against the NFL. What pretext he would do that under, should he choose, I don't know. (The Dear Colleague letter that the Obama Administration used on the enforcement of title IX comes to mind as a use of a regulatory agency that I oppose.)
Another thing he can do, with the help of Congress, is decide to sign or veto a proposed law that affects the NFL's bottom line depending on whether the NFL toes the line he wants them to.
He can also use the social power of the office (the oft called bully pulpit) against the employers of the players, or the players themselves.
We'll have to see how far this goes to be sure, but I don't put it past him to use any of the above, or things I didn't think of, that are available through the use of his office.