@roger,
A lot of the time they are doing it for search engines to see, and actually want nobody on the forum to see it (that way the spam links live longer).
But I'd not try to figure them out, they are spammers, not the brightest bulbs in the shed and aren't going to make a lot of sense. All user-posted links on able2know have a "nofollow" attribute (also known as a "link condom") that basically tells search engines to ignore it. So the smart spammers ignore us and the ones we get are really just wasting everyone's time, including theirs.