I can understand why the author views this as pranking. Innocently applied, I think it is, well, just that. The illegality of man-in-the-middle stuff is probably not that set in stone, though I don't know what the legal grounds are for arresting people who do things like use other people's wifi, and what precedents are set in case law on the matter....

But I very strongly agree with djs_tx. It's bordering on morally irresponsible to create something this automatically capable of hijacking connections and man-in-the-middle-ing it.