I'd imagine those users are not deleted because Bungie does not want their name being free to use once again. Also, so their offending posts are never visible to the casually viewing public. Imagine a reealy long thread from a few years ago. Like Duardo's forum titles thread. If someone spammed that thread just after it was created, and if that user was deleted, we'd have to look at all their crap once again when ever we open that topic. Keeping the accounts censored takes care of that problem without much of a mess.