keeping people from doing whatever the hell they want with it after they buy it.
I can't use an HTML compiler since I only produce pdf ebooks (and certainly wouldn't use anything of Cory Rudl's - he's way over-priced on everything he produces.) My ebooks are behind a password protected directory, and cannot be accessed without a password. Passwords and usernames are assigned by my cc processor.
So, it's not like they can just go to my site and download it.
I still have to check my records, but I think this guy purchased it, and then decided to distribute it. It may be that he thought he had resell rights - I don't know.
I do know, though, that there is no fool-proof way to protect yourself after the purchase. Yes, there are some compilers that require you to enter the password each time you use it, but that's just annoying the reader and the vast majority of people who purchase are reasonable and honest. It's the one or two snakes who sneak in that you've got to watch for.