The reason we have cultured buttermilk, is because the farmers used to let the milk set out for a few days to separate the cream. It is easier to pull it off of cultured milk than fresh. This helps with goat milk also.
When you leave out raw milk, it cultures and tastes about like cottage cheese. It does not get nasty.
If you leave that same milk in the fridge for a week, it might end up nasty - because different bacteria cultures at low temps than at room temps.
If you pasteurize the milk, then it will DEFINITELY turn nasty instead of culturing, because you killed all the helpful little microflora that create good buttermilk.
This is also the best way to make chevre - the flavor is much tangier if you let the milk set out just until it separates (you'll see clear yellow fluid in between the milk solids). Then make your cheese, and it tastes better.
We've lost a lot of these skills, because of our paranoia about germs.