Alice and Bob are conscientious vegetarians. Alice gets Bob to promise her that if Alice ever considers ceasing to be vegetarian, Bob should offer her the most powerful arguments in favor of vegetarianism even if Alice doesn't want to hear them. Years pass, and Alice's vegetarian fervor fades, and she mentions to Bob that she is considering giving up vegetarianism. Alice then says: "Please don't try to convince me otherwise."
What should Bea do? As a rule, the promisee can release the promiser from a promise. So it seems that Alice's request that Bob not importune her with the arguments for vegetarianism overrides the promise. But Bob promised to offer the arguments even if Alice didn't want to hear them. It seems that this was a promise where the usual release rule makes no sense. Can a promise like that be valid?
As the case demonstrates, there are times when it would be useful to be able to make promises that one cannot be released from by the promisee. But one cannot infer the existence of an ability from its usefulness: it could be useful for a pig to be able to fly. Still, it seems pretty plausible that Bob's promise is valid.
But now compare another case. During a fight, Carlos spitefully promises Alice that he's not going to get Alice's birthday party even if she wants him to come. Carlos does not, I think, have any moral duty to keep his promise if Alice reaches out to mend fences, releases him from his promise and invites him to his party.
In fact, my sense is that the release from the promise is irrelevant in the case of Carlos. For suppose that Dan, also fighting with Alice, promises Alice not to get her a birthday present. Dan does not, I think, violate any moral duty by giving Alice a birthday present, even absent a release, as long as it's clear that Alice would enjoy the present.
So how does the Bob case differ from the Carlos and Dan cases? I think it's that what Carlos and Dan promise Alice isn't good, or if it has any value it's a value dependent on how Alice feels about it at the time. But what Bob promises Alice has a value independent of how Alice feels at the time.
But here is another kind of unreleasable promise: an authority might unconditionally promise Alice a fair punishment should Alice do a particular wrong. And it is clear that Alice's releasing of the authority is irrelevant. If what I said about Bob's, Carlos' and Dan's promises is a guide, then unreleasable promises must be valuable for the promisee independently of the promisee's views and desires. Hence, just punishment is good for the punishee.