Why do we feel the need to give every class the exact same thing that another class gets. "Unfair" is not a good enough reason, when you ignore the differences between the two classes in the first place.
AD's for the most part do DPS. That means, that damage they do, imagine healing as lowering their dps by 25 damage for each strike they do. That means, they're starting from an uphill battle, widdling down the Target that's being healed, strike by strike. Mages on the other hand, don't start at the bottom of the hill. They nuke 3/4th's, up to the entire hill, and then walk up the rest.
Mages don't need healing reduction to get an enemy to low health quickly. AD DPS do, since they don't burst as hard.