Jumbo, when a passenger is offloaded, the baggage MUST go too... I believe the practise came about after a lot of aircraft bombings back in the 80's (Lockerbie, anyone?) where passengers(terrorists) 'missed' the flight... now the nutters just bring the bag on and stay with it, but it's done to minimise the risk of the would-be bomber who just has to check a bag in and run... I believe it may even be law. Hope this helps explain it for you.
I do not believe this is the case any more - at any rate, BA have knowingly flying with my baggage on board, but without me, even with me informing them 4 hours before takeoff that they had put it on the wrong plane! I suggest it is just lip-service to safety, as incidents like this rather make a mockery of any supposed safety concerns the airline might have.