If Rabbi Akiva is correct that even previously-betrothed women are covered by the seduction law, then why does the Torah bother specifying "who is not betrothed"? The phrase seems unnecessary if it does not actually exclude anyone.
The Mekhilta answers: the phrase is "extra" — it exists not to limit the law's scope but to serve as the basis for a gezeirah shavah, a verbal analogy with a parallel passage. The same phrase "who is not betrothed" appears in (Deuteronomy 22:28), which deals with the rapist's penalty.
By using identical language in both passages, the Torah creates a bridge between them. From the rapist passage, we know the penalty is fifty shekels. From the seduction passage, we know the currency is silver shekalim. The gezeirah shavah transfers information in both directions: "just as there, fifty — so here, fifty. Just as here, shekalim — so there, shekalim."
The result is that both the seducer and the rapist pay fifty silver shekels. Neither passage alone specifies both the amount and the currency. Each provides one piece of information, and the shared phrase — "who is not betrothed" — allows the pieces to combine. A seemingly redundant qualifier turns out to be the mechanism that determines the precise financial penalty for two different sexual offenses. The Torah's redundancies are never truly redundant.