The problem is, pasturized milk doesn't sour right.
Unpasturized milk was always allowed to stand and sour somewhat before making butter, and this is still done to a small extent. Sour cream was naturally soured in the same way, and for some cheeses the milk was allowed to sour. These products were generally safe and healthful.
Pasturized milk has the natural flora burned off, so it's a bit dicy as to what kind of sour you'll get but it generally won't be healthy. On the other hand, it's generally not a serious health threat either - but could be under unusual circumstances.
