Back when you could steal from monsters that had back packs there was a certain amount of unhappiness from hunters when the monster they had killed had no loot because a thief had stolen it. I'm assuming it was not desirable that the same situation arise again.
At one point I stole a night sight potion, on killing the monster I stole it from I found a second night sight potion in the corpse. I haven't actually tried using these yet, I was trying to complete the set.