I've been using the Internet heavily for the past 15 years or so. I use it as the source for pretty much all of the information I consume whether it's about gaming, work stuff, news, history - anything (yes, even porn) and I have never in my life come across these kind of sites, which "normal" users apparently "stumble" over all the time.
Maybe I need to get out my tinfoil hat but I really doubt whether they actually exist or whether it's all just made up by the likes of the IWF. If it was SO common then I'm sure I'd have stumbled across it in my time.
Also, why would a pedo put pictures and videos up on websites anyway? Surely they'd use forums or peer to peer? Setting up a website would imply they were trying to make money but from the description, the content was freely available so where is the motivation? It makes no sense.
The only plausible reason I can think of is that these sites are all actually honey pots set up by other agencies.
In any case I agree with other commentators here about there being a disproportionate focus on sexual abuse, to the extent that violence with no sexual motivation is increasingly seen as "ok". Very dangerous.
Also it seems like police only care about porn and not even those who commit the crimes. In any chavvy area you'll see 20-odd year old guys with 13 year old girlfriends but nothing gets done about them.
What a crazy world.