That's Disqus (YC S07). Its "reactions" piece pulls in comments from sites like HN, Reddit, and Digg into the comments widget it puts on a given blog post.
I think it was when talking to Jonathan from Plagiarism Today (http://www.plagiarismtoday.com) that I was told that forum posts should be regarded as the authors' own, legal copyright property. I may have some old notes on this around that I can dig up.
Some forums (and sites like YouTube) write some Terms of Use/EULAs to waive their content rights to the site owners (not for any nefarious reasons), although I doubt that these "agreements" hold up.
If I had any comments or forum posts that were copied verbatim in a manner that upset me, I would definitely pursue it legally. But I'm a fanatic like that.
It's useful in that it lets you know that there's a conversation going on somewhere else when you're looking at the comments on the page. It might be enough to say, ``active conversation as of x [unit of time] ago over here: [link]''