Think about this logically, why would they do such a thing? Many NMs are super tough, like King Behemoth or say Aquarius, if using a WS to kill them nerfed the drop then it would be extremely hard to kill them, especially in a close call, as well as pissing players off which is the last thing SE wants to do intentionally. Plus, why? Think about it, why? It takes strategy out of killing certain NMs too.