You are talking in the logs in the referer field, right? I noted this a while back. It seems there are some blog engines, forums, etc that have some reporting exposed, they show the referer from hits... its seems to be an odd form of and attempt at SEO, but I noted as well that I have no such report for them to hit. I'm guessing, like ever other spammer, they don't bother to look and see who it is, they just spam.
The other bit is that if you go to the site, to see what the hell it is, they get a hit... not that there are a ton of webmasters out there, but if you chance to post a question about it, include the link and that gets bot-scanned, etc.
I swear, its a farking arms race. I wrote my own SMTP server and it they keep doing odd shit. I've seen messages with just an image (no text) so you can't scan the text for links. I've been spammed through an exploit with a bounce off NASA's servers. Its like a wack-a-mole game from hell. I finally just added a subnet blackout and I don't even open the port if the request is from africa or asia.
I have yet to see decent subnet blocking in forums... but I suspect its a growing need.
"The right to be heard does not automatically include the right to be taken seriously."
-- Hubert H. Humphrey