There is an article entitled 'The Implications of Unbelief: Tracing the Emergence of Distinctly Shi'i Notions Regarding the Food and Impurity of Non-Muslims', by David Freidenreich, published in 'Islamic Law and Society', Vol. 18, published by Brill. This traces the development of Shi'i fiqh, and makes the point that the particular development of these rules developed in the 5th/11th century; i.e. the matter was much less clear prior to this era. It would require a study of the socio-political background to the development of this fiqh, in order to understand further what brought about this development. There are also a number of contradictory narrations in Shi'i sources regarding these rulings.  Incidentally, Hindus also have rulings on purity with regard to non-Hindus.