This is from the NFL’s 2017 Rulebook; Section 4 Equipment, Uniforms, Player Appearance; Article 8 Personal Messages:
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)….
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.
This is the spirit, if not the letter, of the NFL’s pretended attitude toward on-field player political statements like attacking our flag and national anthem.
Plainly, NFL management has no standards it considers important enough to enforce.