<p>This, to the best of my knowledge, is actually <a href="/questions/tagged/by-design" class="post-tag" title="show questions tagged 'by-design'" rel="tag">by-design</a>. In <a href="http://meta.stackexchange.com/questions/43019/how-do-comment-replies-work">this</a> MSO thread, it says that as you could only be replying to the person that asked the question (in your example) then the @Username is stripped out automatically. If you can fully understand that answer in its entirety I would be more than impressed as it's not exactly clear.</p>