I don't know why the fuck pg thought a whitelist for subdomain preservation was a good idea. It's actually more work than just stripping off a leading www, which would accomplish everything we actually care about.
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..