Since the Rules permit you to deem your ball unplayable in a bunker (as the announcers suggested Woods do), what exactly are the options? The first option (Rule 28a), provides the player with the availability to proceed under stroke and distance, and play a ball as nearly as possible at the spot from which the original ball was last played (the place where the stroke was made that got you into the bunker). This more times than not, will allow you to come out of the bunker (but typically a much greater distance from the hole).
The second option (Rule 28b), allows the player to draw a line from the hole, through the ball, and go backward (away from the hole) as far as he wishes on that line, provided he keeps the ball in the bunker when dropped. The third and most commonly used option (Rule 28c), allows the player to drop a ball within two club-lengths of the spot where the ball lay, but not nearer the hole, and also must be in the bunker.
Knowing that if he dropped the ball in the bunker, it would likely plug, Woods decided not to invoke any of the options under this Rule, but rather to play the ball from its position in the bunker. When he played the stroke, the ball bounced off the wall of the bunker, and came to rest in an even more precarious position along the back-left edge in the same bunker. At this point he again has the option of deeming his ball unplayable (Rule 28 allows a player to do so anywhere on the course except in a water hazard).
However, there is a slight change if he were to invoke Rule 28 for the second shot from the bunker. In this case, it would likely not be wise to do so now because the only option that would have afforded him the opportunity to come out of the bunker (option 28a - stroke and distance), would now place him back in the bunker where the previous unfavorable position was. Woods chose to play this ball as it lay (from his knees leaning over the edge of the bunker), and it was most likely because his options under Rule 28 would not be advantageous to him from this position.