Certain tags don't function as expected with entitydata.
For example, if I were to:
1. Summon a sheep
2. Execute: "/entitydata @e[type=Sheep] {Riding:{Sheep}}"
nothing would happen.
similarly, if I were to run, "/entitydata @e[type=Sheep] {id:Zombie}"
still nothing would happen.
entitydata, you can't summon mobs/change the mob's id, sounds intended to me