In my opinion that's an oversimplification which ignores why copyright laws exist.
Again, I know why they exist: to prevent author from selling a piece of art to several producers at a time, to lessen the competition and to preserve some kind of monopoly for the period of exclusive rights expired.
Everything else is just a rhetoric. One may say they defend the author from piracy. Yes, as long as it helps publishers to keep the monopoly – they do.
It is easy to prove my point: look at the books that are in public domain already. Who cares a moment for which author borrowed what from it, who films what on it, etc? All the ethical moments you refer to suddenly vanish, along with monopoly. Alas.
And as the sons of Feanor did not dare to demand the Silmaril at this point, thus Tolkien 'finds a way' to remove Beren from needing to answer them; or at least he does not set up a scenario in which Beren must decide to withhold the jewel when confronted by any Feanoreans, if JRRT wasn't actively trying to avoid such a scene. You brought up Beren and Bilbo here specifically. Why?
Because they were accused in thieving, don’t you remember?
And the fact that Feanoreans never claimed the Silmaril from Beren, doesn’t mean that they accepted him as rightful owner. They may have been ashamed or afraid of him, but as soon as he died they claimed the Silmaril from his heirs and resorted to arms to restore it. No way they gave up their right on the Jewel for a moment, they just waited for Beren’s death.
The fact that Tolkien 'finds a way' to remove Beren from needing to answer themis obvious. And I understand him perfectly, because Beren’s behavior after Thingol’s death is at least questionable, no wonder Professor avoided this questions: why hadn’t Beren returned the Silmaril, at least for the sake of his children and grandchildren’s safety, for the sake of elven unity before the inevitable Morgoth’s invasion? Bril’ova had to invent the prophetic skill given to Beren to explain his behavior, otherwise it lacked any logic or common sense. Don’t know how it is in English fandom, but in ex-USSR there is plenty of Feanoreans’ admirers who believe Beren to be a bad guy and a thief. And they sound just like you: “So you think that if he worked hard and risked to take the Jewel from Morgoth, it’s OK to keep it from lawful owners?”
In order to somehow show that Tolkien is on your side with respect to copyright issues?
You see, in his books someone who desires to keep something in his exclusive property forever is either a bad guy or a good guy committing terrible fallacy. Morgoth loved the creation he took part in and tried to own it – thus, he became a common enemy N1, Feanor loved his creation more than his soul and gone mad, his boys gave oath to return the Jewels by all means and committed three kinslayings, Thingol was enchanted by Silmaril and Nauglamir and you know what happened, and what happened to the Dwarven king you know too. Celebrimbor wanted to preserve beauty and splendour of Valinor in mortal lands – for the elves only – and was betrayed and killed by Sauron, Sauron himself – everyone knows who he was and how he finished, Thorin stirred a war, which could end in complete disaster unless Bilbo intervened. Be Feanor not so jealous, keep he the Silmarils in Tirion – there would be no chance for Morgoth to steal it. As for me, epimyth is clear. There would be simply no space for such a discussion, if CT just said that he considers the option of selling rights to company A or company B. If author‘s heir wants his fee – it’s understandable, it’s lawful, it’s human whatever high his price may be. We have film, he has money, everyone’s happy. But if he just wants to keep saying ‘no, nay, never…’ This I cannot stand.