Don't you get it? Remember how the Kyuubi's eyes looked like when it was being controlled by Madara. It had sharingan eyes. That's why the Juubi has Rikudou Sennin's eyes/eye. It's being controlled by him.
Originally Posted by Dagoro
Its logical to assume that RS did not have the RG before he became the Juubi jink, and that the Juubi was indeed the catalyst that sparked this doujutsu ridiculousness that has consumed the manga.
In order for the Rinnegan or the Sharingan to awake inside your body you have to be born with it.
Well I found a pick with Kyuubi being controlled with the Sharingan eye and one without:
The one with http://www.onemanga.com/Naruto/398/01-02/
The one without http://www.onemanga.com/Naruto/399/10-11/
What do you think?