Custom loots do not drop when mob is not killed by a player

I am making a tower defense map, and i want the mobs to drop custom loots.
The problem is that they drop only their basic loots when they are not killed directly by the player.

To check this, try :
/summon Sheep ~ ~1 ~ {Equipment:[

{Count:1,id:258}

,{},

{Count:1,id:304}

,{},{}],Attributes:[

{Name:generic.maxHealth,Base:1}

],DropChances:[1.0f,0f,1.0f,0f,0f]}

(all in one line)

If you kill the sheep with a sword, you will get the custom loots. But if you kill it with /kill, a dispenser, a wolf, etc, you will only get the regular loots.

EDIT : Of course, when a mob is not killed by a player, he will drop, assuming the gamerule doMobLoot is set to true. I have tested this in 1.5.2, 1.6.4, 1.7.4 and 14w08a. This bug was already in 1.7.4.