Cookie Monster makes it hilarious, but ... that is, honestly, some shitty writing. How the hell is this happening? There are tons of better erotica available.
It's happening like this:
Small Time Author Launch (ie smaller press, e-press, self publisher)
1) Usually, the only people who have previously read the work are a few friends/beta readers, your editor, and maybe your agent and publisher (if you have those). They're the only people who can start telling other people that it's awesome and "spreading the word."
2) If you're lucky, you'll get a few advance reviews. They probably won't all be on high traffic sites.
3) You might arrange a blog tour. Again, this probably won't be on particularly high traffic sites.
4) If you're very lucky, your publisher might advertise you on a website or print venue, or you might even stump up for this yourself. For the majority of small time authors, this doesn't happen.
5) You could do a lot of blogging and networking, but you're an unpublished author, and your main access is to writers, not readers.
Basically: you rely strongly on word of mouth, but like most debut authors, you're starting small.
P2P Author Launch
1) Your work has been on a high-traffic fan fiction site for a reasonable amount of time, where it has gained enough readers for you to think it worth pulling to publish. In some cases, the numbers can be in their mid thousands.
2) When you put your work on sale, a lot of these readers will rush off to buy it. They'll review it on the vendor and on GoodReads. And they'll tell all their friends. It's the promotional equivalent of a ten ton truck crashing into your living room: very hard to pass by. Your inflated Amazon chart listings perpetuate new sales to readers you probably wouldn't have tapped otherwise.
3) P2P fanfic is a controversial subject, so people start talking about your book online. It all creates a buzz.
4) What with all these reviews, reviewers and book bloggers want in. They want to know what the fuss is about. Whether they say good or bad things, they're making the work more visible to the public…all of this is a heck of a lot more promotion and blog space than Small Time Author could hope to get.
(There are authors who post their original fic online to gain an audience before launch. It does work sometimes--I've done case studies--but most often, not to the level of success as Twifics).
Conclusion: Books like this started their "original fiction" lives with a massive leg-up that most smaller time authors can't hope to get, and they got that by using someone else's work to make theirs more appealing. Whether the P2P fanfic is "original" or not, it invoked somebody else's character names for its own benefit and ends up reaping the rewards at the expense of the original fic author, who is slowly pushed further down the Amazon foodchain with every new "successful" P2P. (We could suggest these P2Ps are doing good things for various genres, but a) I don't think we have figures for that yet, b) the above would suggest otherwise, and c) on the "people who bought this also bought" lists, if it's Fifty Shades, they tend to not be buying anything else in the genre, and with other popular Twifics, they seem to be buying other Twifics as much as anything else).