[/entitydata] Certain tags don't function as expected.

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.