It's just the way the Kindle is linked to the Amazon library, I think. The book doesn't have to be DRMed.
Given that it's quite easy to accidentally buy things with One-Click, I kind of suspect that there would be an uprising if they didn't offer this.
On the other hand, the goodwill is probably worth more than the losses from fraud. (And if you're that keen to avoid paying for a book, there are plenty of other easy ways that leave less of a trail.)