How do they serve those customers? On a platter?
Here are their top advertisers:
Another example: http://www.ftc.gov/opa/2007/08/edebit.shtm
Then you've got CPA Empire which is run by Scott Richter a notorious spammer. Their offers are extremely spammy/scammy. There are a lot of people making millions of dollars (well, not as much now) by feeding on the sub prime market. This was especially prevalent during the housing bubble as you could sell mortgage leads for $40-100 each. Auto loan and payday loan leads are/were also extremely profitable for affiliate marketing companies.
As those sources of revenue dried up the companies that were still around switched to recurring revenue models where they'd sell crappy products on free-trials and then bill the consumer the full price for a couple of months worth of the product before they had a reasonable time to cancel the subscription.
Facebook and a few other places stopped allowing advertising for those products and the merchant account providers stopped handing out "high risk" accounts like candy so much of that revenue has dried up as well. Not sure what the latest scam is. If you dig into it though you'll find that a lot of these offers are run by the same people. One company goes under and they just come back out with a new name and scam six months later.