Luke writes:

I'm curious. To those r.b.t participants who operate bicycle shops:

I understand a satisfied customer is essential to a prosperous
business relationship. It's to your benefit to have him riding and
enjoying his bicycle as much as possible. And to that end, the right
equipment and good service are requisite. But at what point do you
continue to serve an obstinate customer knowing that to satisfy his
wants is at cross purposes to serving his needs? Have you patently
refused to entertain a customer's request in order to, in effect,
protect him from himself?

As Jesse Ventura observed, "you can't legislate against stupidity."
All the shop owner can do is warn the buyer that this product is not
well suited to the purpose. Most customers will heed your advice; the
ones in Lycra will heed what they read on the Internet or in bike
rags.