I assume that you are not required to display the % for a specific item if the drop rate of all item of the same rarity is the same. Since it would be a pain in the *beep* in order to maintain the list if that is the case since drops change every now and then and you can give a decent estimate if you know roughly how many item of each rarity are available if you really wanted. But if items of the same rarity have DIFFERENT drop rate, then perhaps they would have to reveal it.
Just my guess anyway, I'm too lazy to dig up the law and read it...