Them's the rules
Robert Griffin III, quarterback of the Washington Redskins, was reportedly told by the NFL not to wear his t-shirt with the phrase "Know Jesus Know Peace" on it to a post-game press conference because the shirt wasn't a Nike product. RGIII turned the t-shirt inside-out to avoid being fined.
The NFL prohibits players from "wearing, displaying, or otherwise conveying personal messages either in writing or illustration, unless such messages have been approved in advance by the League office." Griffin was fined $10,000 in December 2012 by the NFL for sporting Adidas attire at a press conference.
The Comcast Sports Network in Washington, however, pointed out that other players came to the interview podium not wearing Nike brand clothing. Redskins linebacker Ryan Kerrigan, for instance, was wearing a t-shirt by the Five Four Clothing Company.