Spiders are only powerful if you show them fear

Really just a guess, but I believe a lot of butterflies are brightly colored to indicate that they're poisonous/taste bad. The spider might just be annoyed by the butterflies and want them gone, instead of actually wanting to hunt them. It might also be a species that feeds exclusively on another sort of insect, like flies, so it doesn't have the instincts to try and hunt the butterflies. Very neat interactions regardless!