Robert Griffin III marches to the beat of his own drum. Dude is an independent thinker, for better or worse. That's why the criticism of Mike Shanahan running him into the ground last year, while it was a fair criticism, was a bit overplayed because Griffin was just as culpable in that situation.
Griffin has shown a distaste for the Reskins' coaching staff's strategy of easing him back into competition, indicating he was promised that he'd be allowed to start on Week 1. And Griffin doesn't care much of what people think either.
So when the NFL requires its players to wear Nike apparel because of its contract, it shouldn't come as a surprise that Griffin The Contrarian would come out there going all Run DMC on everybody. Griffin is an adidas guy, and the adidas t-shirt he was wearing was more about him rather than the team whose jersey sports the Nike check.
Too bad the league could care less about Griffin's independent streak. The NFL fined him $10,000 for the unauthorized gear, and the regular season hasn't even started yet.