So my question is, what is the inherent difference between our interpretation of crop circles as being intelligently designed, and a creationists interpretation of biological structures as being intelligently designed?
The most inherent difference is that crop-circles do not come about through descent with modification. Life DOES come about through descent with modification. Someone else mentioned a house. This applies as well. For a new house to come about, do we need two other houses to mate? Besides being silly, I think this illustrates the point well. We don't need an outside force to produce life. Life produces life.
Through our study of evolution we know that changes happen through mutation. We have never seen a limit to where and when mutations can happen. We have observed that beneficial mutations are passed on to the next generation in greater numbers than neutral or detrimental mutations. Through this process, complexity and design can appear. None of these mechanisms apply to crop circles, and therefore the comparison is not worth the pixels it is written in.