As a dairy scientist with a Minnesota farm background, I'm often asked why people pay more for milk labeled according to the way it's produced. One example is the recent controversy over labelling milk from cows not supplemented with rbST.
I've observed that it's not really about any differences in the milk. When I speak with consumers who pay more for milk that is labeled as rbST free or organic, I've found that they do really know that there is no difference in the milk. These are casual conversations, and not a part of a scientific study.
These people are willing to pay more because they think that dairy farmers who don't use rbST or who produce organic milk are somehow better for the ecosystem or environment as a whole, and they often say that they are willing to pay more to help support those farmers. Of course, farmers who choose not to use rbST receive only a very small fraction of the price difference.
It's ironic that the recently published research study from the Proceedings of the National Academy of Sciences clearly demonstrates that rbST-supplemented cows are more 'environmentally friendly.' Cows supplemented with rbST have a smaller 'carbon footprint' than do those raised under conventional management.
An authoritative article about the safety of rbST supplementation for both cattle and humans is available for those interested.
In the interest of full disclosure (very important these days) I have no financial connection with Monsanto.
Hope that helps.