The reason that these scams are so successful is that there is real need for professional SEO services. But there's simply no way to separate the wheat from the chaffe. (Or is it that there is no wheat? I can't always tell). My family's small business is a perfect example. We sell beads and jewelry. Nobody actively working at the store is a techie (although they do things with pliers and tweezers that would make your head spin). We know that our website is barely serviceable from an SEO standpoint and frankly, we could probably use some professional help.
As the "designated techie" in the family (which I am decidedly not), the family forwards me an SEO solicitation at least once a week. Of course, I patiently (or not so patiently) explain each time that it's a scam. But the fact remains that the family knows we need this type of service at some level. So the temptation to believe that there's someone out there that actually understands this stuff better than us (which shouldn't be hard) is so very great. But it's tough when the entire industry appears to be a scam. Sad, really.