There's a link at the bottom of the wikipedia page. It says WARNING- SOUND!, so be forewarned.
http://en.wikipedia.org/wiki/Peter_Popoff
Ordinarily I would agree with the natural selection theory, but that's easy enough to say when you're healthy. If you're just home from getting a terminal diagnosis at the doctor's and you're more scared than you've ever been, I could see the vulnerability. Or your well-meaning grandma drinking in the miracle water in hopes that something good will happen to you. I don't know why this guy didn't disappear for good after Randi exposed him.
Edited to add: in that article I first posted, they interview a woman who sent him a total of $350 over the course of...I think it was a year. The "free" water only works if you keep it under your pillow, drink it first thing in the morning, then write him a check for $17. He sends you some miracle salt, which you're supposed to eat over the course of a week. Then, if you send him $35, a miracle will happen. And so on.