This is one of my pet peeves.
There are, I think, two reasons why one wouldn't allow hardcore imagery on a free site's index page:
1. The list is owned by someone who didn't really understand the link list-free site game before he or she decided to open their own link list. This person had heard it rumored that hardcore isn't allowed on a free site's entrance, so he or she included that on their rule page.
2. The list owner is someone who, for whatever reason, believes that a free site's entrance page is of some great importance.
Holy land. And though they own a link list with thousands of links out to these tiny porn sites,
they don't want to link to porn. And they fear that children, or the easily offended, will stumble upon your free site's index in the SE result pages, apparently assuming that no other page from the free site will be indexed, and
it will be their fault. Then, the FBI - or worse! - will bust down the door to their 1972 mobile home and drag them off to Federal [fuck me in that ass] Prison.
Hardcore imagery on your index/warning page is just fine.