I'm guessing it's not there for higher levelled accounts due to the fact that there might be a very huge number of posts (and thus resource intensive) to remove via an inbuilt command?
I'm guessing it's:
1. meant to not give all moderators the powers to remove any account (all Mods can nuke Newbies but only a few can ban anybody)
2. based on the assumption that spamming Newbies' posts won't be missed in a discussion, while removing all posts of an old member can leave "holes" in discussion threads.