Pyramids without a point

Every time I see a spam email or spam blog comment hawking someone's freeiPods.com/freeiPodMini.com/freeMiniMacs.com link code, I wonder why people fall for pyramid schemes time and time again. I'm only 32, but I seem to see these scams pop up every couple years (Herbalife, Amway, etc) and people just keep falling for them every single time.

I wonder if it's one of those natural laws, like pi being around 3.14 or the natural log of trends curving around 2.71. Maybe the conditional reinforcement of pyramid schemes hits some magic sweet spot where just enough people at the top actually get something while every sad sucker beneath gets diddly squat, and the ratio of rewards to bupkis is a perfect number.