First off, it's not "his" airplane and he had no business putting the sticker on it. I wonder how he would react if a member of AA senior management slapped a "Screw APA" sticker on his car.

He just may have violated an FAR. Engineering drawings show what exterior markings are supposed to be on the airplane. I doubt that the sticker was on the drawing. An airplane that is not in compliance with the applicable engineering drawings is not legal to fly. I know it's a nit-picky point, but that's the FAA's stock in trade.

While I think the penalty was way beyond reasonable, what he did was rather silly.