True. It's kind of inconsistent if you think about it. Why is it that only someone with the Rinnegan could summon it from the moon (it was fine when we thought that the Rinnegan was merely from Uchiha + Senju, but because it is Ashura and Indra; it is because of that power, or Hagoromo's (The Rinnegan) that it was summoned.). Well, then why is it that Obito with just Uchiha + Senju could control it? That makes no sense with what we know now. Nagato and Madara both had some of Hagoromo's power to control and summon it, but Obito could do it with just Uchiha + Senju. hmm
You need a rinnegan to break the seal, once that seal is broken it is available to anyone with sage chakra to use.
Remember that Madara is the first incarnate to awaken the eyes, all the previous ones failed.
When the term uchiha and senju comes in play, it basically means uchiha senju chakra.
Obito had Hashirama's cells attached to his left body, giving him Ashura's chakra.
Madara put the black rods into Obito's body too giving him Madara's chakra, in other words a piece of Indra's.
Since he is not a reincarnate, he won't awaken the rinnegan but he can still use it's techniques through the black rods (chapter 606 page 14-15) as a substitute.
Although he won't be able to harness the full power like Limbo Hengoku.
That's why in the cave Madara was able to teach him the six path Justus, because he was using his chakra (black rods).
He really wasn't lying when he told konan that he had both Madara and Hashirama's chakra in him.
