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?
we do not assume that whoever made the crop circles broke the laws of physics to do so. it's great to say "look what god made!" but entirely another to say he operated outside of naturalistic processes.
also, it cannot be positively concluded that all crop circles are man-made (or technically that any except those filmed were). but we know that people exist, and that they are capable of creating these things. further more, we know that people walk around with boards and strings and little loops on their hats: we know HOW they do it.
apply this to creationism for a second. we don't KNOW god exists, or space aliens exist, or whatever. and if it's not evolution this intelligent designer used, we sure don't know how they did it. further more, it's totally contradictory to observations.
so it's an interesting analogy, but it doesn't really work.