North, thanks for the clarification on the store bought yogurt and kefir. I have made my own yogurt from store bought, so I know that the cultures are active and you're correct that pasteurization takes place BEFORE the cultures are added to convert the products to yogurt and/or kefir.
In order to use the NYA logo and make the claim "live and active cultures", the products have to contain a minimum of 100 million cultures per gram at the time of manufacture.