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Why is it called butterfly...
the japanese term makes it very clear that you can easily enter the corner at the 3-3 point. (I have heard similar "closing the door" metaphors from Chinese and Koreans for playing a third line move after e.g. a high shimari, then it is obvious that spending three moves to build a door is bad.) Butterfly on the other hand is based just on the appearance with no implied lesson to be taken from it, isn't it?
John F. You'd have to ask Charles Matthews why he (allegedly) called it "butterfly" but on the face of it this sounds like confusion with the generic term tonbo = dragonfly (there is kotonbo, Kannon-biraki and ootonbo).