Well ... when an OUT decision is challenged, the first thing the 3rd umpire looks at is whether the ball was a no-ball. If Broad had walked, it wouldn't have allowed for the possibility that it was a no-ball and we still can't be sure if the ball was actually valid.

Broad had a perfect right to allow someone to decide whether he was out or not. If it had been an uncalled no-ball then he wouldn't have been, would he?

:-)