Because DK's literally have no cleave. Their "cleave" is an AoE ability which is hardwired into their standard rotation, giving them free AoE damage at no dps cost - more targets simply means more damage for them. This also means their "cleave" output will be somewhat low, while their AoE output will is immense. It's not exactly overpowered(it was stupidly overpowered when Howling Blast did its full damage to all targets hit - that was changed to half damage to all targets apart from the primary target though).
The Warrior cleave doesn't seem overpowered at all. I've yet to see Warriors pull Combat-like numbers.
What cleave do Mages have that needs fixing? I'm pretty sure Combustion can't be spread to more than 3 targets now, which is the only "cleave-ish" ability Fire has. Is there something I'm unaware of?