Wiki has a useful page on this, PDQ. Basically, the growth of the "pro-biotic" cultures is greatly effected by temperature, and moisture. Low temperature and low H20 levels keep the bacteria quiescent until ingested.
I am sceptical of the benefits of such yoghurts and drinks myself.Even should the bacteria survive the acid bath en route, it is difficult to see how such minute quantities of bacteria would materially affect the flora of the lower intestine...
I'm not if they're of any benefit in a healthy digestive system, but it might do some good in upset systems due to antibiotics or such.
I used to hatch discarded/abandoned bird eggs from friend's and our own aviaries and had little success getting the babies through the first few days until I started using neat natural yogurt for the first day and gradually lowered the % until 14days. The Lactobacillus acidophilus is most active at 37° and certainly helped to breakdown the sugars of the food and promote floral growth.