A rule is a tool for the diffusion of conflict. It is an impersonal external reference point, which eliminates the emotional dimension of a dispute between individuals. This impersonalisation of a dispute assists in the creation of order, stability and tranquillity. It does not ensure the creation of the same but merely prods the social organism in that direction.
A retrospective law does not appear fair or just because it fails to some degree to provide an impersonal external reference point. A nagging suspicion that the rule has specifically targeted an individual or group of individuals for the behaviour introduces a personal dimension to the rule.
The personalisation of a rule occurs when the rule is intended to target or perceived to target an individual or a group. The rule then does not operate as a rule and acquires the character of an order (with the accompanying lack of moral legitimacy) directed at an individual.
This personalisation renders the rule questionable. If rules were, to begin with, intended to diffuse conflict and secure tranquillity, any rule that doesn’t impersonalise the conflict must surely be questionable in terms of its pedigree as a rule.