Only Wither Skeletons drop their head

Only Wither Skeletons drop their skull. I think that's a bug, i don't understand why creeper/zombie/skeleton heads shouldn't be obtainable in survival gamemode.
What I expected to happen was...:
At least 1 on 200 zombies drops its head
What actually happened was...:
Only 64*3+35 rotten flesh, 4 iron ingots, 2 potatoes, 1 carrot, leather pants, gold boots
How to reproduce:
Kill a lot of skeletons/zombies/creepers: no heads will be dropped.