Again Ace mate, this one's in your head, not the game. The whole point of the passive field is that it just sits there. Any occasional asteroid attacks from a passive are accidental (I've seen maybe two or three in all my time with FS2).
Now, ye of the very long handle, if you read the help file it will explain the virtues of the inner and outer asteroid box. Setting an inner box inside an active asteroid field will create a safe zone where ships will not be attacked. As V put it, this is handy for hiding a station inside an active field

Oh, and I'm liking this term 'attacking' for asteroid impacts. Must remember to shove it in to a mission somewhere
