But thats not entirely true.
Ignorance and blind trust, open up people to attacks in any number of ways. For instance through the subconcious, subtle attacks aren't usually gaurded against, in that respect, how many people can tell the difference between nightmares and sleepless nights caused by an external entity or by themselves? Most people expect to have nightmares at some point, its a part of life.
Or Seances? There disbelief may protect them, but not their homes, how many stories have you heard of seances gone bad?
Not to mention I've met people who have been caught in the cross fire of warring Wiccans and I know the Occultist still fight amoungst themselves, and how many people have found themselves psychiclly attacked by workmates?
Any one with knowledge of the Art can use it agianst others, belief or not, and this is especially true when it comes to working in magickal circles.