I'm not sure if this is intentional or not, but familiars have Weapon Proficiency: Creature, though they do not have creature weapons. At least the imp does not. Tried to cast Flame Weapon on it, but there were no claws/bite/slam attack or anything for it to absorb the evocation onto. Can't say about the other familiars though.
This is given to them so that they do not get AOos everytime they attack something.