The process of producing decoy shapes isn't really a problem. Why these spiders made the shapes is more interesting.

I think that the spiders that didn't create these decoy structures were selected against in the evolutionary process. The production of decoys could have been genetically engrained into their instinct (via mutation), or, this species really are actually able to recognize itself and create a decoy -- both of these cases would have led to less of them being eaten by predators, hence natural selection.

I can't wait to hear the results of the study after they research it more closely!