For everyone that says Tiger "knew" he was in violation of the rule, ask yourself this question. Is it possible he thought that his original shot was also in the same line as the balls path going into the hazard. I think that's entirely possible and you have to give a professional golfer the benefit of the doubt. In addition to that the rules committee reviewed the incident prior to Tiger signing his card and came to the conclusion he did not deserve a penalty. At the time of signing his card it was clear to him, and the rules committee, that he was in no violation.
Now after the fact it was discovered that he had dropped illegaly. The reason he did not get DQ'ed is that he signed a "correct" score card at the time of the signing. As stated above, both him and the rules committe agreed no penalty was worthy. It wasn't until a "viewer" called in and brought it to club officials that he may have known he violated a rule, which can not be determined. The Rules Committee, R&A, and USGA all agreed with the ruling. It is obsurd to me to think a player would withdraw given that circumstance.