O'Reilly had the serious flaw that you could just claim you own a book by adding its IBAN to your account's library and thus being able to "upgrade" it and obtain the ebook version for $5. I wonder why that worked as long as it did.
Because enough people would still buy them at full price, and if you were willing to scam your way into a $5 ebook, at least you're paying something for it instead of just pirating it.
I always saw it as a way to turn pirates into paying customers, and the would-be pirates still felt like they were getting a good deal. It's a win-win.
