An interesting angle... Digging it up again because I think I can add something.
My first thought was something others have already mentioned: the "created by an intelligence" hypothesis simply makes much more sense for crop-circles because we can readily identify good candidates for this "intelligence": human pranksters. They are around in abundance, they have the (proven) ability to create circles and the (proven) tendency to create pranks. We do not have as easily an "intelligence" candidate available for something that looks like biological design.
Secondly, and I haven't seen this pointed out, the "intelligence" that is being displayed in cropcircles is different from apparent intelligence in biological systems in that it is "non-functional". Crop-"circles" are intriguing because they show geometric regularity, but this fact and its complexity have no identifiable "purpose". This while the reason for claiming ID lies in the observation that everything "works so well together". That there seems to be "purpose", "intent" behind it all. And also that it "couldn't have been much different". There is no such requirement for crop-circles. A completely irregular cropcircle is just as viable and possible as one that shows a complex fractal. The property that makes it look like it was created by an "intelligence" is not necessary. Or still expressed in another way: its "design" stands completely on its own. It is not intertwined with the environment, guided by the circumstances in which the crop-circle exists. It doesn't have to respond to pressures of any kind, it doesn't have to fit into constraints. Well, this idea is actually harder to express than I thought, lol I hope you get what I mean.