Article 8, Personal Messages, page 23, states:
"Throughout the period on game-day that a player is visible to the stadium and television audience (including in pregame warm-ups, in the bench area, and during postgame interviews in the locker room or on the field), players are prohibited from wearing, displaying, or otherwise conveying personal messages either in writing or illustration, unless such message has been approved in advance by the League office. Items to celebrate anniversaries or memorable events, or to honor or commemorate individuals, such as helmet decals, and arm bands and jersey patches on players’ uniforms, are prohibited unless approved in advance by the League office. All such items approved by the League office, if any, must relate to team or League events or personages. The League will not grant permission for any club or player to wear, display, or otherwise convey messages, through helmet decals, arm bands, jersey patches, or other items affixed to game uniforms or equipment, which relate to political activities or causes, other non-football events, causes or campaigns, or charitable causes or campaigns. Further, any such approved items must be modest in size, tasteful, non-commercial, and noncontroversial; must not be worn for more than one football season; and if approved for use by a specific team, must not be worn by players on other teams in the League.”
On another note let me share a story of when I worked for West Marine. I was the District Manager of the Pacific Northwest District (Seattle/Portland). When ever a new employee joined WM they had to read the "Employee Handbook." Typically the Store Manager and new employee (s) would read it out loud which took about 20 minutes. The new employee (s) and Manager signed a sheet of paper that basically stated that they had read the Employee Hand book.
One of the items in the hand book was that since WM required the employee to wear a company shirt along with a company provided name tag the employee could not wear any "bling" on the shirt or name tag without being per-approved.
I was visiting a store one day and the Asst. Manager had a Christian cross on his name tag. I asked him to remove it. When he refused I went into the employee records and showed him that his condition of employment was based on his agreement to the rules in the Employment handbook. He refused to take it off so I suspended him. After checking with HR I later that day asked him once more that if he wore anything on the company shirt or name badge without prior approval bling he would lose his job. He agreed to not wear the Christian Cross.
A couple of days I was in the area and swung by the store to pick up something for my boat. Again the Asst. Manager was working and again he was wearing the aforementioned Christian Cross but on the company provided shirt. I terminated him on the spot. I then called the Store Manager and asked him to come in ASAP as he was on duty now.
I am a Christian. As a matter of fact I am leaving for my Monday night Bible study class in the next 30 minutes. The Asst. Manager violated the rules of the game - So I fired him.
My opinion is, based on the NFL rules, the men who take their knee during the National Anthem should be warned and then fired if they do not comply.
Last, many folks think that Mr. Trump should not have said what he did. Looking at the First Amendment of the BOR's it states that we have a right to free speech (Among four other rights incorporated in the First Amendment) that means all Americans including the President.