One of the purposes of the pardon power is to nullify convictions where a law was broken but the conviction, justified as it might be by the law, was unjust in the specific context.

Now, its designed (on the premise that the law is always just in the general case) to deal with exceptional cases, but it is no less appropriate a vehicle for the case where the law failed to be just in general and thus any conviction under the law is unjust in its specific context.