It is mostly just a scare-tactic used in the media to symbolize a shark. In the wild, animals typically adapt to the hunting method that is most likely to succeed. For example, if a shark were to hunt a bird sitting on the water, if it's fin was up above the water, the bird would see it and most likely fly away before the shark could get near it. If the shark were to come from underneath, the bird most likely would not be aware of the shark until it was too late.