Advertising occurs on this site all the time, and works to great effect, it's just that the manufacturers and retailers don't pay for it - the product users and prospective users do it for them. BPL users talking about products, reviewing products, recommending products to other users - boiled down, fits nicely into the dictionary definition for advertising.
The UL cottage gear industry wouldn't exist if no one ever talked about their innovations. The kind of talk that frequently occurs here - the kind that can convince someone they need to spend $335+ for an 11oz piece of fabric to use as shelter (with an 6 to 8 week lead time, no less) - if that isn't advertising, then just what is it? :D