The Tiger Rule: USGA 33-7

Here it is:

33-7. Disqualification Penalty; Committee Discretion

A penalty of disqualification may in exceptional individual cases be waived, modified or imposed if the Committee considers such action warranted.
Any penalty less than disqualification must not be waived or modified.
If a Committee considers that a player is guilty of a serious breach of etiquette, it may impose a penalty of disqualification under this Rule.

I note that "exceptional individual cases" is undefined. Thus, a decision not to disqualify Tiger is entirely appropriate, as under any number of criteria this situation is surely an "exceptional individual case."  The two-stroke penalty that Tiger was assess comes apparently from Rule 20-5.

For background on the whole situation, see ESPN. The NYT provides a run down of potentially relevant rules.

And here is the statement from The Masters, identifying Tiger's TV interview as the source of information leading to the controversy.