It's more like saying that because I can write a simple set of computerised rules that can cause a spider-web design to be created, that spiders must use the exact same rule-based mechanism to do what they do.
Actually, it's more like saying that:
IF humans could MAKE a spiderweb, and
IF we had no evidence that spiders exist,
Then we could conclude that spider webs are made by humans.
The question is: Who do we KNOW that can make spiderwebs or crop circles? We know that spiders can make spiderwebs so we don't need to look for another cause. Only if we KNEW that humans could make spiderwebs would they be viable candidates for the source of any spiderweb.