because a Warlock in service to a Devil is radically different to a Warlock in service to a Demon
Are they?
The simplified version is, Warlocks in service to a Devil will more than likely be Lawful Evil where a Warlock in service to a Demon will more than likely be  Chaotic Evil
So a Fiend Warlock isn't likely to side with the Goblins. They are too chaotic, it's evil just to be evil, but the Goblins aren't likely to progress any plan in motion by a Devil. But a demon, most definitely. Just for the havoc they'll reek