Why can car insurers put your premium up for things you didn't claim for?

My car was hit by a customer who was driving off without paying for petrol at a petrol station. Just took off a bit of paint so paid for it myself as quote was way lower than excess. The accident was reported the police but plates turned out to be stolen. Whilst getting a quote for new insurance I was asked if there had been anything i'd paid for rather than claimed for. Thought i'd better disclose this as i had reported it to the police. This bit of paint added another ?70 onto my quote! Why ?- i was told it was classed as a liability, how can a bit of paint that was rectified within a week be a liabilty? So not only was i out of pocket for the paint but the insurers are now charging me anyhow. Can they really do this? I would accpet that they could if the car had been structurally damaged, but not a bit of paint work!