If a member has been banned for spamming, the userid will still show up on the members list - mainly as it keeps that e-mail address blocked. If we removed the account they would be free to sign up using the same e-mail again. The way automated spam software works, it's easier for us to just leave spam accounts as "banned" but not removed (they can't do anything like post).
The majority of 0 posters are just people that registered to browse (and perhaps subscribe to threads), or those that may post in the future.
It takes up only a few extra kb of space to accommodate them, so we might as well leave them all.

There are quite a few users that sign in regularly but never post, so I wouldn't want to catch anyone out.