I'd say that it's probably some sort of bonus to attract exchanges into listing GOLD. And this would mean that GOLD team would pay the storage fess for the involved GOLD from their pockets / revenues.
So that's how it is. But from the exchange's viewpoint, they receive nothing other than GOLD holders keep their balance on exchange for free of fees. This also leads to the exchange become an attractive target for hacking since hackers know there are a bunch of GOLD balances stay there.