USGP polesitter and season points leader Vettel's helmet had the words "gives you wings" written on the top of his helmet, which, as the motto for his Red Bull sponsor, goes against a Bernie Ecclestone rule that forbids advertising on top of racers' helmets. Total F1 says that this rule was made so as not to ruin the on-board camera view from above the driver.
Hamilton, on the other hand, was forced to alter his helmet due to the letters "H.A.M." painted on the back. While a casual observer might think this has something to do with his last name, these letters are actually an acronym for "hard as a mother******" which goes against the FIA's no-swearing policy. Both helmets were easily fixed with a little cover-up tape.
Be sure to also check out our Day 1 recap of the USGP race weekend.