Board Thread:Naruto News/@comment-25648694-20141109075459/@comment-1512913-20141109105203

Correction, having Rinnegan doesn't mean you can automatically use all natures. The eyes just make it easier to figure out how to do them. There's no evidence Sasuke can use all natures because of the Rinnegan, he has had it just for 1 day or so.

Remember, he is a prodigy/genius whatever and an incarnate of Indra, student of Sannin Orochimaru and so on. Also the Sharingan can copy and analyze techniques, so with enough intelligence and talent, it stands to reason that Sasuke learned how to use the natures through observation with his Sharingan imo.