SPAM isn't always trying to sell you something . . . at least as it's come to be known.
The old notion of spam was unwanted mails in your inbox. The new notion refers to just about anything automated and unsolicited on the web, whether malicious or promotional.
As for why this is spam or why a spammer would use a forum.
Go turn 'subscribe to this thread' on and see what sort of info you get back when someone replies to your thread. Depending on the forum app being used and the settings on the forum app being used plus the screenscraper and aggregate used by the spammer they can get *a lot* of info out of each reply (or very little).
Not to mention that hitting the site in general gives them the ability to see what sort of numbers a spam posting gives them. Again, all of it fully automated, so there's no real extra work in it to most spammers, so why the heck not try to get more info, even if it's just one email address (that is usually valid if it's gotten from a forum).