Hi Chook,
In my opinion, the issue isn't about the Bulldogs challenging the penalty, there were grounds for appeal on a number of issues including the validity of the cap itself but they wisely decided for the good of the game as a whole not to drag the league through another round of court cases. This is about the punters themselves challenging the penalty.
Whilst a court detirmination of "The Bulldogs previous board were all well aware that the extent of the breach they were involved in would result in the loss of all premiership points (37) and expulsion from the finals series" would result in a simple exchange of cash for loss of bets, interest and costs. Any other result could pave the way for a direct challenge by the punters on the NRL.
e.g.
The court finds that whilst the Bulldogs were in breach of the cap, there is no case to answer of wilful negligence to the punters as the penalty was not effectively communicated prior to the offense as all previous breaches resulted in monetary penalties only and the latest communication circular from the NRL prior to the scandal becoming public was a 4 point for the first $100K then 1 point for each additional $50K in the current season.
Therefore the punters could then challenge,
The penalty itself, the salary cap, the validity of the 2002 finals series in an effort to procure payment of their losing bets.