Majoru Oakheart
Adventurer
The reason is to maintain the AC hierarchy:
Paladin: Plate + Heavy Shield
Fighter: Scale + Heavy Shield
Warlord: Chain + Light Shield
Cleric: Chain
Ranger: Hide
Rogue: Leather
Warlock: Leather
Wizard: Cloth
The idea that, generally, the higher up on the list you are the more AC you have. Warlord was given the shield to make them have a higher AC than the Cleric. This makes them more than a cleric with slightly different powers, it makes them more capable of standing toe to toe with the enemies, without putting their AC in the defender range.
Paladin: Plate + Heavy Shield
Fighter: Scale + Heavy Shield
Warlord: Chain + Light Shield
Cleric: Chain
Ranger: Hide
Rogue: Leather
Warlock: Leather
Wizard: Cloth
The idea that, generally, the higher up on the list you are the more AC you have. Warlord was given the shield to make them have a higher AC than the Cleric. This makes them more than a cleric with slightly different powers, it makes them more capable of standing toe to toe with the enemies, without putting their AC in the defender range.