The rules that I think are pertinent to these current player protests are listed below. I have copied them from the 2017 NFL Rulebook that can be found at
http://operations.nfl.com/the-rules/2017-nfl-rulebook/#pdf-download
Football games are for entertainment. They are not and have never been intended to be a forum for political causes or political activism.
Enforcing the following existing rules is all that is needed to put an end to these controversial protests .
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RULE 4. GAME TIMING
ARTICLE 1. KICKOFF ON SCHEDULEBoth teams must be on the field to kick off at the scheduled time for the start of each half. Prior to the start of the game, both teams are required to appear on the field at least 10 minutes prior to the scheduled kickoff in order to ensure sufficient time for proper warm-up. Designated members of the officiating crew must notify both head coaches personally of the scheduled time for kickoff prior to the start of each half.
Penalties:
For delaying the start of a half: Loss of 15 yards from the spot of the kickoff as determined by 6-1-2-a.
For failure to appear on the field at least 10 minutes prior to the scheduled kickoff of the first half: Loss of the coin-toss option for both halves and overtime, and loss of 15 yards from the spot of the kickoff.
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RULE 5. PLAYERS, SUBSTITUTES, EQUIPMENT, GENERAL RULES
SECTION 4 - EQUIPMENT, UNIFORMS, PLAYER APPEARANCE
ARTICLE 8. PERSONAL MESSAGESThroughout 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 non-controversial; 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.