I'm aware of the Plagiarism Today post as I exchanged e-mails with its author. I didn't link it because it contains inaccuracies (especially wrt SEO, etc).

Individual comments make up a very small percentage of a page's content, so what we're doing is more similar to Google than you suggest (they also index your comments -- search for them). We display excerpts of pages; we attribute them liberally. A search result on Google leads you to the full page of content; a search result on BackType leads you to a full page of content. We index content for purposes of discovery i.e. search.

== Fair use

Also, it's pretty obvious that we're not in competition with content publishers, and we provide a very effective way to prohibit BackType from indexing comments.