I personally think ramming in any situation should be considered a criminal offense, even those by accident going through the letterbox. You wouldn't let someone bump your car and drive away scot free would you?
The difficulty lies in finding out who is the one ramming, and who is the one being rammed. Because from a pure simulation and physics standpoint, both are equal, but from a gameplay standpoint, they may not. There are cases where that may be easy (for example one ship being stationary relatively to the instance' local coordinate system, or one ship moving very slowly and the other very fast), but for every easy case there are 10 or more cases where it's all a big grey area.
And if you have significant fines for ramming, that would just change the method of potential griefing. Instead of ramming someone to death (or hull damage), you can still force a fine upon them.
And bounties for ramming are straight out of the question, because if you have ramming considered as an attack, you can kill any ship in a starport just by ramming them, then both of you would get a bounty and swiftly destroyed by the station.
Edit: I remember a situation which I have experienced myself with NPCs several times already, mostly at starports but also once in an RES: A system authority ship, usually an Eagle or a Viper, comes straight at me, starting their usual crime/cargo scan. I am in a Python, 2 pips engines, 2 pips shields, approaching the station. I was moving in a straight line, minding my own business, but the NPC forgets to decelerate and smashes into the front of my shields, bumps off, tumbles, and explodes. It was definitely their error, I did not even alter my course, but they got destroyed. Should I be fined for their stupidity? What's then to stop a potential griefer to pick a free Sidewinder and boost headfirst at full throttle into someone's T9?