Answer

Some explain the rule that “we do not administer punishments based on logical derivation” by saying that perhaps the a fortiori inference can be refuted. Therefore Scripture goes to the trouble of writing it explicitly, to say that this is a valid a fortiori inference. But regardless of that explanation, the law itself—that we do not punish on the basis of logical derivation, whatever the reason may be—means that when Scripture wants to impose punishment for something, it has to write it explicitly.