True, but in all the time I've played this and all the ships I've killed, that has never happened. If I've ever accidentally shot a clean NPC it's because he flew between myself and my target, not because he flew behind it. From the sound of it, that's also what happened to the OP.
Having said that, I can't remember the last time I got a bounty for accidental glances off friendlies. I can however remember the last time I got a bounty because an NPC flew his ship into the back of me and exploded :\
It does happen, I have been in fights with fixed weapons where I have missed the target and hit a friendly behind the target... that was quite some time ago now though... These days I am lazy and use gimbals