I believe that America's obsession with don't drink AT ALL when you're pregnant is liability driven.
If your doc says you can have a glass of wine or beer a day, which I believe would be fine, and you overdo it and then later when your baby is born with problems you can say, "Well, my doctor said I could drink."
So to protect themselves doctors veer in the direction of "no alcohol at any time while pregnant" mode to avoid a lawsuit.