Dr. Awkward said:
The security here isn't really for WotC. It's for the customer. By paying $1 with a credit card, it can be proven who paid for the book, so if someone guesses at a code and activates it, it can be traced when the book that really contains that code is purchased and the swindle discovered. Since the book will be available on the internet within a week of release anyway, there's really no other benefit. It's not going to prevent piracy.
However, it will mean that the majority of D&D players will have electronic editions of their books. And they'll be constantly reminded that for just $10 a month, they can plug those books into the DDI, which you can't do with pirated books. The more electronic books you own, the better the DDI deal gets. That's why WotC is even bothering to do this in the first place. To get people signed up for their electronic services. The e-books are just a hook.