Why do people think women can't be Vegan while pregnant?

Right now my fiancee is pregnant. She eats very nourishing nutrient dense foods and lives on a high quality organic vegan diet. Right now she's pregnant with our twins (a girl and a boy) and she has been having a very healthy pregnancy. She never gets ill and she is a picture of perfect health. Yet people who don't live like us are always saying she should be eating milk and dairy and she is depriving the babies of "nourishment" and that she is a "bad" mother. Why are people so unwilling to accept ways of living that are different from their own?