It is their constitutional right, but only your employer can define what you are free to do on company time without consequences.  I know the Broncos want to keep this low key, and would likely be the subject of a lot of unwanted attention if they told the players publicly not to do this at a game.  It's challenging, now, because unlike Jerry Jones, the Broncos did look the other way and murmur platitudes about freedom when they didn't have to.  Undoubtedly they thought it would be a one-time thing and that they'd be in the clear once Marshall did the intelligent thing of talking to police and working with organizations that actually do something, not symbolism over substance.  Now they are in a precarious position, and frankly I think that Elway needs to have a private talk with his employees about what is expected in the business he's running.