Well I'd say the main reason for this is being that Black Hat methods make things easier. People, and society today, are always looking for an easier way to do things.
There is a lot products and so you can sell, which is not blackhat products. I think it's just in forums, where you're having a community with a lot members, then they have a bigger chance to sell something the other members can gain some profit on, rather than selling a product that is just useful.