But said content does not belong to you. You aren’t saying anything, someone else is Quoting is even worse, as you lack any justification for fair use. And it has been argued before that linking to a site is essentially the same as quoting. You need a legitimate reason to do so, or you are committing copyright infringement. While this has never been proven, remember that the SDMB aspires to higher than normal journalistic ethics (which is why you can’t quote a whole work where most places would let you.)
Usually it is done to make their search results appear higher. Like, since the SDMB often appears quite high in Google searches, the pages it links to will also appear quite high. It’s like using our website which has a good reputation to vouch for one that doesn’t.
And why do they care about their search engine rank? Because the higher it is in the list, the more people are likely to click on it. And even if the site itself is not an advertisement, it often contains an ad that will get the person revenue. Or maybe even proving people use the site will keep it from getting shut down by someone else.
Anyways, even if the SPAM post is a blatant advertisement for a product, the person sending it may not be advertising as much as trying to help someone out. For example, if someone has a thread about how hard it is to use Office 2007’s interface, I can link them to a place where they can buy a product that will make it more like the older interface. A spammer could also provide a link, but the motivation would not be to help someone, but to shill the product. And it can be hard to tell the difference, when you have real people writing the posts.