Because someone can use such words (not a spammer) and will be banned because of this.
From my point of view the safe system is that when a new member post something will wait the confirmation from administrator to approve it or not (if it is a spammer will be banned; if it is a user, the comment will be approved or something like this; anyway if are lots of spammers and adminstrator can't do all the process then the system after a period, maybe 2 weeks, will banned the user because the confirmation was not approved by the administrator - I named this system auto-cleaning).
In principle it is something simple and no one will be harm in the process but I don't know if it possible to implement this on this forum.