Google "comment spammers" to learn a little more about what this is about. They run around looking for any likely programs that might be susceptible to taking comments and hopefully, auto-posting those comments. They will attack blogs, forums, contact forms, articles with feedback, k2 in joomla, anything that shows promise. They're interested in promoting their crap, don't leave any stones unturned. Maybe they can get a cheap link back to their pharmacy, porn, etc site. Maybe some just do it because they can.
In addition to what Graham said, some use programs that are built to get around front-end comment forms and post directly through processing scripts. That's why in some cases they seem to be successful in spite of captcha protection.
Edited by bobmeetin, 21 October 2013 - 07:26 PM.