Hm, let's say instead of a dog it's a child that ran away from home. How would the situation be then in your opinion?
Now you could say animals are not children. But for some people they are. And there is such a thing as neglecting parental/supervisory duties, which is comparable to neglecting leash laws.
In both cases, what it comes down eventually - in my opinion - is the cruel statement "Dude, your dog/kid ruined my car."
Legally domestic animals are not the same as children; the comparison is not valid. If they are morally equivalent to you personally and you would prefer to take the loss on the vehicle, that's fine. But the OP is not "cruel" for wanting to be made whole for an accident that was not his fault and could have been prevented by reasonable measures on the part of the pet owner.