Why do spammers spamRichi wrote a pretty good article with mathmatical estimates that shows the answer to the question and the solution to the problem. Don't get me wrong, these people are somewhere below pond scum on the morality scale, but somebody is clicking the links because I keep getting more and more of it.
Stop clicking on the links. And refuse to buy from companies that you see advertising this way. It is all about the $$ so you have to vote with yours.
On a similar topic, Richi gives us another pointer to another tool to avoid. Challenge/response is not only not a solution to the spam problem, it is another problem that has been created by spam.