I think the main reason why some services would advertise in the forum is because it can't advertise elsewhere.
I do not agree. The only reason any seller advertises his site or service on a platform for long time if it is converting well for him. I am very much sure that these advertisers are getting good returns with the investment they are making with the signature campaigns.
I didn't said the that they do not get good returns. But they most likely lack options to advertise in places where they would get better return.