Scion said:
It not being fair sounds like the same person who would complain about getting hit by the fireball when he was invis, or someone stepping on a trap which hits him as well, or someone easedropping on your conversation and hearing whatever secret plan is being discussed.
I still think the fairness issue is unclear.
Fairness is if someone gets what he feels that he or she deserves. Unfairness is if someone gets what he or she doesn’t deserve or does not get he or she feels that she does deserve.
Cleave+AoO is not truly unfair from a character's point-of-view, but from a player's point of view. As said, a rogue PC may inadvertently set off a trap which damages another character, which the other character may think of as unfair, but it is not unfair to the player. Why, you ask?
It is unfair from a game-rules point of view. As atom crash pointed out, the rules form a framework, a kind of player-DM contract. It is like playing Risk with the additional rule that you lose an army whenever the player sitting on your left hand loses two. Maybe such a new rule is not unbalancing, it just feels unfair, because it breaks with the general premise of the Risk ruleset. If you ever played Magic and Magic Unglued, you know they try to be fair in Magic, but they are not fair in the Unglued variant. In Unglued you can lose because you wear jeans.
Thw whole DnD rule framework is built on the premise that you can only place yourself in danger. An unfair spell would be a spell that kills a enemy if it fails his save, but kills your closest ally instead if it makes its save. (Note this is not a comparison to AoO+Cleave, its an illustration of the unfairness of the rules).
Actually, unjust would be a better word than unfair, since injustice is unfairness according to a certain set of norms, in this case the 3.5 ruleset.