Herd mentality? Trust?
Enough people will trade their potential to purchase a jetski for a UPC code because they have a belief that jetskis, like boats, are a hole in the water in which to pour money. Their UPC code, however has demonstrated value, and potentially may increase in value, to so many people, that the risk seems worthwhile.
Besides, if you really want to be out on the water, don't want to piss away money each season on batteries, maintenance, and outrageous insurance and registration fees, you'd be better off picking up sailing or windsurfing.