Good point. It's not very effective at all.
But from their point of view, it's free advertising. And if only one out of one hundred spam post ends up in somebody clicking the link, it might be worth their effort.
From my point of view, it's annoying garbage that litters forums all over the internet. Some are very quick to delete such posts, while other forums don't seem to care. The problem with not caring is that spam that doesn't get removed just invites more spam. Some forums get overwhelmed with it.