I don't see how this exposes liability for you—is Google liable when people leave their printer's/router's/fax's/whatever's web configuration interfaces on the public internet with no password? I don't believe they are, and I've seen a number of Google searches in the past on the Hacker News front page linking to pages and pages of them.
However, like Google, you probably want to have a quick way for people to remove their site from the index once they've discovered and secured it.
I, for one, would love to see a search engine of public rsync servers.