As forum moderators with years of experience behind them can attest to, there are a lot of spammers on the interwebs. As Roger Montti of WebmasterWorld says, there are three different types of spammers from noticeable ones to the not-so-noticeable spammers. Here's his breakdown:
1. Users who bulk-spam forums for links. Their links are usually poor quality and they simply have no idea how to market themselves or their sites.
2. Amateur spammers who try to "spread the love" about their site and write related forum posts about their somewhat-related-but-not-really blog.
3. Users who think it's acceptable to post links that only point to their own blog posts about a newsworthy topic. Sometimes that content is good, so those posts may not necessarily be removed.
You know what they say, though -- you need to network in order to be noticed. New bloggers assume that it's safe to start commenting about their site on every single forum post so that the word spreads. (Instead, I'd argue that you should write interesting content FIRST with a signature for your site on forums that allow such a thing.) After all, it's a lot easier to detect self-promotion. On a personal note, I had a guy comment on a blog post with his "amazing" video on Twitter, complete with made-up testimonials. I sternly told him that it's not allowed, and he responded to me saying that "you act as if I'm selling something." (He was selling something: his content -- himself.) The point of forums is to participate in a community and not to abuse the audience and assume that it's acceptable to shove content down their throats that they may not necessarily have an interest in.
It's an interesting discussion and one that you may have your own thoughts on.
Forum discussion continues at WebmasterWorld.