Clerics have the option to choose ranged attack powers based on wisdom. Warlords do not have ranged attack power options, they must be melee fighters, and as such, they get the option to use a light shield.
I don't understand why there is so much crying over 1 point of AC difference between the two leaders. It's not like you'll have people running around with +5 shields as in 3.x.
Classes are balanced in many dimensions. For instance, keep in mind the Cleric is less MAD than the Warlord. You can build a Cleric focused purely on Wisdom, or purely on Strength, and function reasonably well. Warlords need a good Strength, *and* they need good Intelligence or Charisma.
In a similar vein, Paladin's are more MAD than Fighters, so this could be one reason why they have Plate where fighters don't. Or it could be because Fighters have these cool reliable powers, whereas Paladins don't. When you are comparing classes there is tons to consider.
I will agree with one of the comments though. Fighters could have gotten shields as an optional class feature of the defensive build, and they could get something like +1 damage when wielding a melee weapon two handed as an optional class feature of the offensive build. It would seem to fit nicely with the other class designs.