The FDA is an overly oppressive agency that is stealing away our freedoms in a way that the average person can't relate to. However, this particular case seems to demonstrate how wrong they are in a way that anyone should see.
If you're tl;dr...
In a nutshell, the FDA is telling Diamond nuts that they can't tell people that the omega 3 fats in their nuts are good for your body in the ways that omega 3 fatty acids are commonly stated to be good for you. If they do, then the walnuts are classified as a drug.