DJ said during his press conference that he did not agree with the committe ruling but it did not effect the outcome of the championship (his score changed but it did not change his order amongst the group).
If Johnson disagrees, then he signed an incorrect scorecard. Right?
Wrong. Up until now, at least, the player's score was the the number he signed for on the scorecard. If he signs for a lower score than he is deemed to have had, he's disqualified ... but if he signs for a higher score, the higher score becomes official. So signing for the higher score and accepting the penalty [even if he feels he didn't do it] makes it all hunky-dory. /sarc
[Incidentally that is how Roberto De Vicenzo lost The Masters in 1968 ... he was really tied, but signed for a 4 on a hole where he made a 3, so he was deemed to have lost by a stroke.]
DJ could have refused to sign the scorecard at all, on principle, but there are not many players in the world who would risk disqualification, right after winning their first major championship. Maybe Tom Watson.