This site encourages the sharing of info within the community, but the sharing of info and spam are two different things. How can one tell the difference? Usually by getting to know the member and how they participate within the site determines if it is spam or not. For example Brian may have a link in his signature but he has never tried to promote his site. Brian often posts links to his ebay auctions in the for sale forum, and he then donates a percentage to apug if the items sell (which he doesn't have to do). And Brian is very active in the apug forum community in a positive way, also contributing his time to help me out if I need an extra hand. It's safe to say if a member like Brian posts a link for any reason it's not spam..
There are some people who do not want others to know of another site that has free info.