I think it's safe to say Miley did not grow up in a particular culture or environment where twerking was common. I also think it's safe to say that she is putting on this "twerking persona" (for lack of a better phrase) as a way to make money. It's part of her schtick now, and it wasn't before. That is why it is offensive, because this is a portrayal she has the privilege of putting on and taking off; that, as a white woman, it is celebrated when she puts it on, and she does nothing to acknowledge this; and that she is harnessing pieces of an oppressed and stigmatized culture in order to make money for herself and feed her own ego. What she's doing is the definition of exploitation.
http://groupthink.jezebel.com/solidarity-is-for-miley-cyrus-1203666732
http://groupthink.jezebel.com/solidarity-is-for-miley-cyrus-1203666732