How could it possibly be worth it to sit down and write this drivel, send it off to a gazillion people, and get one hit from a guy who has to try a pair of shoes on before he'll buy them?
Who talks these people into wasting their time like this?

They aren't doing it to get hits. The drivel is text taken from articles that are relevant to the link and have valid context. At that point, they post on boards that have a reasonable PR or will provide a decent chance at some PR juice being pointed their way for the anchor text, and when google spiders the site, sees the linkback and increases the ranking of their site.

Wouldn't they make more money stealing scrap metal?

They pay a company in India $.02 per recaptcha solve/signup to a board. The email address used goes into a catchall, a bot uses a proxy server to authenticate. Once they have authenticated, an automatic process posts to one of the busier or forum with the most posts.

Short of the captcha being solved, there aren't any humans involved - even the article text used is scraped from the web using the terms they are trying to rank highly for. Then, once google spiders it, their receptor site gains ranking and relevance and starts to show up higher in the results. Then, champagne falls from the heavens, doors open, velvet ropes part.

Being automated and all makes sense, and it's good that somebody in India gets 2 cents as part of the deal, but I'm betting they don't sell a lot of shoes.

It is possible that the company was hired to do SEO and drive traffic to the site and has nothing to do with the site. It is also possible that once it gets good rankings, they will sell the traffic to another site.

In order to profit from ppc or cpa spam, you need a mule site. These guys generate the traffic, find some unsuspecting guy in a niche that they have cornered, offer to partner with him and bring him a bunch of relevant traffic. All he has to do is split 40% of his revenue with them. They might even let him set up a shadow site so that he only has to split traffic they bring in. They bring their real clicks plus their botnet clicks, he starts to see revenue, sends them checks like he's supposed to - until google shuts him down at which point they find another mule site.

Many different ways to use botnets for profit. Since spam is getting much more difficult, they are transitioning.