The clause is there to provide a tool to deal with some of the interesting things that people will try and use a CDN with no bandwidth charge for. i.e. what if

The clause is there to provide a tool to deal with some of the interesting things that people will try and use a CDN with no bandwidth charge for. i.e. what if whole movies were given a different content type and file extension, would CloudFlare suddenly be the world's largest CDN of pirated movies? Most such questions have been tried already, the clause allows us to answer "only for an exceptionally short amount of time".