Datamining is just a computation, an arbitrary computation. It has input value(s) and an algorithm which depends on computed intermediate values and finally an output(s). There is nothing special about the data that datamining works on which differentiates it from any other kind of data within that framework I described. This is the wonder of homomorphic encryption. It DOES let you do aribitrary computation without decrypting the data.
That's not the same as doing arbitrary computation on data whose general semantics you are totally ignorant of. Is it numeric data? Is it text processing? What is its format. Sure, you have to know that level of detail but that's like saying "this string is a pssword". Does it get you any closer to knowing what that specific encrypted password is? It does not.