This is a prime example of why the (domain.com) section at the end of links on HN should be the full hostname, and not just the top domain. I only looked at this because I thought it was a Google posting :-)
That said, if I had to guess a motivation for the technique, it'd be to stop people getting attention by wildcarding subdomains and linking up nonsense like click-here-for-my-awesome-blog.domain.com, etc. Not that such a thing should avoid getting flagged anyway..