It's very different from cleaving on your own turn. On your own turn, you choose the target. Cleave allows you to make an easy kill so fast you still have time to redirect your attack to another legitimate target. In effect, the cleave feat makes the time taken to drop the first target negligible as long as there's another legitimate target to switch to. When an AoO opportunity comes up, should you elect to take it, you don't choose your target at all, you only get to target the creature who provoked the AoO. Outside of the character's turn, there should be no other legitimate target to use cleave on.
To look at it from the point of view of the target, a character should not be subject to AoO unless they drop their guard and provoke one themselves. They should not become vulnerable because someone else did and went down.