Why are fermented dairy products pasteurised?
Probiotic products are generally pasteurised, then the desirable cultures are introduced, in a similar way to yoghurt. There may be exceptions but those you can find in the supermarket are all made this way. 

There are several reasons. Shelf life is a fairly minor one, but a batch contaminated with a disease-causing species would be a problem not just for health but for the manufacturers profits. Another is consistency. Even a benign bacterial/yeast culture can produce off flavours, or even just different flavours from different strains of the same species.