(Exodus 22:15) introduces the law of seduction: "And if a man entice a virgin." The Mekhilta explains why this verse is needed when the law of the rapist is already stated in (Deuteronomy 22:28-29), which requires the rapist to pay a penalty (knass) to the father.
One might reason that if a rapist — who acts against the woman's will — pays a penalty, then a seducer — where the woman consents — should certainly pay a penalty as well. The logic seems airtight: if the more violent crime triggers payment, the less violent crime should too.
But the Mekhilta explains that logical reasoning alone is insufficient to impose financial penalties in Jewish law. Even an airtight a fortiori argument cannot mandate a payment that the Torah does not explicitly authorize. The Torah must state the seducer's penalty in its own right.
That is precisely why (Exodus 22:15-16) exists. It establishes the seducer's obligation independently, without relying on inference from the rapist's penalty. Both crimes generate a payment to the father, but each has its own textual source. The Mekhilta insists on independent authority for each financial obligation, even when one could be derived from the other through impeccable logic. This principle — that penalties require explicit Torah authority — prevents the expansion of punitive law through judicial reasoning alone.