One of the site policies, as is the case with just about all internet forums, is "lease do not advertise other sites."
This is the real life equivalent to plastering public walls and telegraph poles with billposters. It's not actually meant to happen but it does anyway.
Except the moderators here are far more effective at actually bringing them under control. But don't worry, we won't actually make you take them all down, we just lock the posts.