Apparently the reasoning behind it goes something like "you have had an accident and therefore statistically, you are more likely to have another accident even if the first accident was not your fault"
Another aspect that strikes me as unfair is that while they are quite happy to count your accident record in your car against you when quoting for insurance for your bike, they are less willing to allow your eight years of accident free motoring in a car to weight the bike quote in your favour!