Why are fermented dairy products pasteurised? Could it be for longer shelf life?How much longer does pasteurised dairy products last compared to non pasteurised one?It doesn't make much sense to me to destroy useful bacterias.I understand that pasteurisation destroys harmful bacterias, but probiotic (unpasteurised) products do exist (how do bottles of probiotic beverages not explode?).
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.