Legally would depend on the terms of your lending agreement.
Generally a lending agreement specifies the principal, and payment schedule (payments, and term period). Not all lending agreements allow pre-payment. If pre-payment is allowed the lending agreement will specify how pre-payments affect the payment schedule (reducing the future payments, or shortening the term period). Most agreements actually have both clauses to ensure the minimum payment does not go below some minimum ($10). Some lending agreements will also have similar terms allowing you to miss payments.
I think "they're doing you a favor", "they're aggravated", and raising it as an ethics question assumes far too much on their part. It's a standard contract they wrote up to be competitive with other institutions, and attractive to a wide range of borrowers. In a general sense they likely don't care about you whatsoever as long as you fulfill your obligations under the lending agreement.
Ethically it is completely open-ended. "They" are professionals from a dozen different specialties who have studied and continuously improved this thing for years from legal, financial, statistical, risk, sales, ethics, social, marketing, and every other perspective. They have meticulously crafted a system that offers borrowers flexibility to make both "good" and "bad" choices.