That's the downside to running a site with open registration. It used to be the case that spammers would write a computer script that would then be let loose on the web, automatically registering and posting spam links on websites.

These days, most sites have fairly robust anti-spam software that prevents this kind of automated abuse, so the spammers have turned to the third world, and will pay individuals a dollar or so for every 1000 sites they register with and subsequently spam.

They're not really expecting you to be interested and to follow the links. It's more about trying to get round Google's algorithms - the theory being that if 1000 sites suddenly link to a certain destination, Google will rank it much higher in their own search results, making the site in question easier to find find for those who are using Google to find porn.