An owner of a private forum can opt for a forum where he/she can feel free to promote his/her services, books or products.
In that case, the next text is added to the top of his/her forum:
Personal Forum: self-promotional messages by the forum owner are allowed
That is a kind of warning to the forum visitors. Expect a messages that could be defined as "direct promotions, conflict of interest"
Why do curezone allows for Commercial forums?
Because curezone visitors like the advice they can get from people who have books/services or products to sell.
People who have books/services or products to sell would most likely leave CureZone after a few weeks or months, unless curezone allows them to promote their own business here.
Them leaving CureZone would be negative for CureZone visitors who appreciate the advice they can get free of charge.
So it is a compromise.
In order to have people with knowledge return to CureZone every week, curezone must give them something in return.
As we can't afford to pay them for the large number of hours they invest into answering questions on cureZone, the next best thing to do is to allow them to promote their books/services/products inside their forum.
They help CureZone visitors free of charge, while CureZone allows them to advertise free of charge. You scratch my beck, I scratch the computer keyboard.
Some of those commercial forums are between the most popular forums on CureZone:
so there is no question that those people are respected by curezone visitors.