From what I can gather, it is because as Secret Keeper Bill told Ron where it was because Ron stayed with Bill and Fleur after he left Harry and Hermione. From what Jo explains in Deathly Hallows, when a Secret Keeper tells another about a place the secret is passed on to them, so they can also reveal the secret. It does contradict what has been said about Secret Keeper's previously, though, and is a bit of a grey area.