Stuckey - There is no official rule that says the challenger has to beat the champ definitively. Thus, judges are under no obligation to follow it. In fact, they would be wrong to apply such reasoning because its not in the rules and it involves predetermined bias.
If some fans really think it should be an official rule, then petition commissions to adopt it. Otherwise, stop citing as a reason to justify the champ getting the nod in a close fight.
Each round is judged independently, and no judge should have a bias against a fighter based on anything other than what happens in the each round.
Lets say that one of the judges had the fight 2-2 going into round 5. What if he gave the round to Aldo, but then said after the fight he actually thought Edgar won it, but since the fight was so close he gave it to the champ, citing the unwritten rule that the challenger must beat the champ definitively.
This unwritten rule, if applied, is unfair and can easily make already sketchy judging even worse.
I do believe the challenger has to BEAT the champ to win, not just sneak a few rounds out and hope you pull it off.
BTW I thought Aldo won the fight regardless.
To be the man you have to beat the man! This is an old, unwritten rule. Welcome to combat sports 