They already do: a comment is a response to the blog. You can post the response on the blog, which is moderately public, or you could email the author, which i

They already do: a comment is a response to the blog. You can post the response on the blog, which is moderately public, or you could email the author, which is more private. You could write a response on your own blog, which may be more or less private depending on whether you actually have readers, and you could conceivably go on television and broadcast your response to the whole world. There's a difference between the kind of information (response), and the method of communication (comment on a blog).