Considering how many different subdomains google.com has, it's probably a good addition to said list (I hadn't noticed that some subdomains are shown - e.g. wordpress.com - nice catch).

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..