I fail to see how people would get fun out of making some dumb ad in a forum where no one is going to care about it at all.
Well, couldn't they at least teach their spambots proper grammar?
If those people were affected by spambots they wouldn't like it either. At least that should deter them! Why don't they care about other people's feelings?
Because they are terrible people, or because they find building spambots amusing and don't mind being affected themselves much. There are many of these people in the world.
So, what do spambot-making people get out of it?