ProphetofDoom - in this particular scenario, the fact that the rabbit wasn't insured is not the issue. Had the rabbit been insured (and as other posters have already pointed out) the insurers would chase the OP for recompense, as it was the OP's dog that caused the injury.
I don't insure our pets, instead choose to put money into a separate bank account. If an accident happened, or a fox got into the run and caused injury, I would pay whatever necessary. In this case, it was someone else's dog that caused the injury to this rabbit. Therefore, the rabbit owner is not responsible for the cost of medical treatment, but the dog owner is, as it was her dog, which was clearly out of control.
I can't get my head around why some people appear to value a rabbit less than a dog. A rabbit is a chosen pet. My children loved the rabbits (and gerbils, and hamsters, and cats etc, etc). Why do you think of a rabbit as deserving of less love than any other pet? 
I don't insure our pets, but instead choose to pay if and when necessary. On the other hand, if we had a dog, I would insure it, simply because as the dog's owner, you are ultimately responsible for any damage or injury that dog might cause. Dogs are more likely to cause damage or injury than any other pet. Our rabbits only damage our lawn. Cats are not a problem with any of our neighbours, as we all have cats, however, if one of our cats injured someone's rabbit or guinea pig, I would not hesitate to pay for the vet's bill. Our rabbits are unlikely to cause injury to someone else's pet - they are contained within their run and are not unusually aggressive - certainly not enough to cause injury to another neighbour's pet.
As far as dogs are concerned, Third Party Insurance at the very least, is probably advisable. If the owner chooses not to take out such insurance, the owner is still responsible for the cost of any damage that dog has caused.