Reading (and discussing) a log on Dioxides has led me to believe that if you raid a cabal to retrieve a cabal item, you receive 500xp even if you are not a member of the cabal whose item has just been retrieved.

Example: I am a Maran. I group with a svirf warrior who is not a member of the Fortress. The Empire has the orb. We go raid Empire. When the Vanquisher falls and the orb is returned to the Fortress, I receive two sets of experience points, first from killing the mob (0 at hero I believe) and then 500xp for a successful counter-raid. It seems that the svirf non-fortressite also receives that extra 500xp, despite not being a member of the fortress. There is no counter-raid for him, no cabal item is returned for him, nothing in his personal world has happened.

Is that intended or a bug?

They helped retrieve the item. They get the exp reward.