[mjw1007]

Then, presumably, if the criminal doesn't follow the contract the case ends up in civil court.

And if the criminal refuses to submit to the judgement of the civil court, that's contempt of court which needs to somehow be enforceable.

You could make the contempt of civil court a criminal matter, but then the same insurer ends up having to pay restitution to itself and this doesn't seem to end well.

Or if the civil court has its own enforcement arm then we don't seem to have moved enforcement to the place he thinks we have.

[RobinHanson]

I did say explicitly that the one exception to fines as official punishment is failing to arrange for insurance. Your contract with an insurer can specify the court that judges disputes with it, and the forms of punishment available there.