Requesting insults

Okay, picture the situation: you're browsing in your favourite bookshop and a book catches your eye. You've been looking for an introductory text to a particular topic and this book fits the bill. Happy that you've found it, you tuck it under your arm and saunter over to the counter to join the queue. Then, while you're patiently waiting, some complete stranger yells at you "Hey you! You're a complete idiot!". Now how does that make you feel? Angry? Confused? Outraged? Indignant? I certainly don't think I'd be happy. So why would you go and buy a book titled "The complete idiot's guide to..."? Or for that matter, "...for dummies"? I find it curious that people don't like it if you insult them for free, but are quite happy to pay good money to be insulted - and again every time they look at their own bookshelf. But they do, and whats more, they do it in their thousands. Craziness.