if it didn't pay, people wouldn't do it. I don't know how it works, but surely it must. I know enough about human nature to know that people like money and most (if left to their own devices) are lazy.
Part of the "revenue in a click" is your ip address showing as "live" for future targeting purposes, I'm afraid. For the same reason never ever click on a link in a spam email, or respond or a spam text or call - you are worth more as a proven living person than as just a speculative target.
I'm surprised the spammers even get to register... I can't recall, does the site use a CAPTCHA for registering, and, are there any antispam plugins in use? (think of akismet for starters, which has a plugin for forums but not sure about vbulletin)