unfair on the teams that have a bad tourny
But cancelling an event is unfair to the teams that are currently lower in the rankings. If you double the score of the last event, then everyone knows going in that the score is double. No one could have known when playing the first three events that there would only be 4 scores instead of 5.
Or, put another way, cutting the number of season points by 20% isn't fair to the teams who HAD a bad tournament either.
And even counting the last event double is still an advantage for teams in the top of the rankings - they only have to defend their spot once instead of twice. It's just less of an advantage than cutting out one of the last two events entirely with no score adjustment.