Of course these sites should be cracked down on, and cracked down hard.
However, I've heard that the police's targeting of users is fairly indiscriminate : credit card being used? Investigation or at the very least a caution (without making it clear that a caution is a criminal record). Doesn't matter if your credit card was stolen, doesn't matter if you're using a credit card on one of the (legal) sites on the same web server.
Some detail from basic googling here : http://timworstall.com/2009/06/21/operation-ore/
Operation Ore, not so perfect as the media presented.
Or, if that's not enough for you, how about putting ../ in a URL and getting raided by armed police? The guy still has a criminal record for that, I believe.
The thought of being subscribed to a small payware site of any variety that's been hacked to serve illegal material should send the shivers down the back of everyone. I do not have confidence that the attitude of 'it's a site we've never heard of, therefore they must all be guilty kiddie fiddlers' would not be prevalent. All it needs is one article by a moronic tabloid journalist to ruin the lives of innocent people ('Emperor Beetle study site found to be secret kiddie porn ring' on page 1 and then a correction four weeks later on page 23 'plenty of people actually innocent, only interested in beetles')