Because Looting enchantments seem to add a looting chance rather then increasing it relativly, mobs created with the Dropchance feature drop items when killed with a Looting enchanted item even when the Dropchance equals zero.
This is only a bug or more of a disturbance when the Dropchance is intended to equal zero, it doesn't matter at any other higher dropchance.
Duplicate of MC-2958.