Did some research on vikings and Nordic cultures and I have the theory that the developers based the class of barbarians on vikings. They use swords and shields, but the main weapon used for wars and raids were axes. Developers were accurate when it came to putting a penetration attribute to axes.
For vikings, swords and shields were honorable weapons to fight with. They also used bows (mainly for long range only; they switched to melee for close combat), spears, and backup knives. With all this in mind, I think there is a good sense of logic for barbarians using swords.
Don't know too much about Paladins, but I do know that maces are often magical weapons used in gaming and mythology. Paladins using axes don't make sense at all.