Yes, when you submit a comment to HN you intend for it to be published on the HN site. However, using comments for text mining purposes is a different thing and in principle consent is required to do that. The default case is that user submitted content remains copyrighted by the author, with an implied consent for the site to publish the material on the site. There can be an explicit license such as with Wikipedia or StackOverflow; in that case text mining may be explicitly allowed.