When the infraction of the ball drop occurred (The Masters) I personally felt Tiger got special consideration given the penalty others had received for signing an incorrect score card. Here is an example I recall that was similar to Tiger's ball movement in that the movement was so small that Padraig didn't realize it had happened. He was disqualified for basically the same infraction Tiger was.
http://sports.espn.go.com/golf/news/story?id=6044348 It is unfair I think that Tiger gets a break and Padraig (and several others if you look) gets disqualified. I can't think of any reason for the difference other than Tiger "moves the needle" (read Sponsor $$).
There is another explanation: the rule requiring DQ in that situation was modified in 2012 to give the committee latitude where a penalty is assessed after the player turns in their card.