A little bit of history from the wiki:
In 1098 DR the Barbarians from the south, seeking to plunder the wealth of Peltarch, began full assaults against the city walls. Both guilds, being more apt to fight at sea then at land, became little more than bit players in the conflict. Up until this point, defense of the city had been mostly private individuals and groups with a vested interest in protecting their products. The Senate, realizing that a catastrophe was in the making, passed the Guardian and Defenders Act in an emergency session. The bill created two branches of the military, the Guard for threats at home and the Defenders for threats from abroad. The private town militia was quickly drafted into the new governmental organizations and set to the task of turning back the barbarian threat, the final blow coming at the Battle of Nars Pass.
After the battle, the city of Peltarch was faced with the moral dilemma of an overwhelming number of Prisoners of War captured during the battle. The barbarians had abandoned their disgraced comrades and refused to negotiate for their freedom. The Senate was reticent to free the men for fear they would simply regroup with their tribes to attack again. The Guildmaster of the Seafarer's at that time, Ruith "Red Blood" Snydders, convinced the Senate to grant him possession of the prisoners and dispense them as slaves. Washing their hands of the whole affair, the Senate granted Snydders request, in essence condoning the slave trade in Peltarch. Snydders took several of the more brutal and loyal men and used them as his own personal bodyguard. The rest he sold into slavery to the town of Hoarsgate and some whispered rumors that some slaves were even sold to the Giants of the Giantspires. The Seafarers, having tapped into a lucrative monopoly in human chattel, refused to stop with the sale of the prisoners, and began the dirty business of buying and selling.
While many in the city opposed slaving, foremost the Wavebreakers, it was over a hundred years until the practice could be stopped due to the Seafarer's immense influence on the Senate.
That ancient history but it does go some way toward explaining the fate of the barbaric Nars tribes.