I think troll warlocks are in the same boat as orc mages. There was a small class imbalance and the designers and writers thought "Why not?". Which is the easiest way to answer this: "Why not?". Orcs have warlocks so they do have the comprehension to wield the arcane and become mages, and again trolls have mages and shaman which we have been shown can be corrupted into warlocks.
It's one of the few new class/race changes that doesn't break the lore over it's knees.