I’m not saying I believe this interpretation myself, but it is not quite that cut-and-dried. If you personally put all the sand on the beach (because it was otherwise a rocky beach, say) then it is your sand. This is even more the case if you put a lot of work into making sure it was very high-quality sand. You would be well within your rights to not invite people to your beach house, or to ask them not to melt your sand and take it home. Of course, at this point the physical-object metaphor breaks down.
So let's do a different comparison: intellectual property. That should make everything simpler and not in any way cause any disagreements. One could regard the Silmarili as stand-alone fanfiction for the light of the Trees. The current real-world uneasy truce regarding fanfic is (simplified) that authors have widely varying opinions on it, publishers tolerate it, and fanfic authors don’t try to sell it. But suppose that somebody wrote some (for-profit) fanfiction of something that was relatively unknown and the fanfic got big. The original author would be justified in wanting credit and probably royalties and would justly feel cheated if the fanfic author claimed the entirety of the work. See: Candy Crush and CandySwipe for a real-world concrete example.
What they did is similar: Fëanor took something of the Valar’s work, made something else out of it, and then said “This wholly belongs to me, and my children who weren’t involved at all, and we will kill you if you say otherwise”. And just like in the real world, nobody bothered with who exactly owns what until the stakes got much higher.
This all hinges on how much the Valar have a right to the light and how much this gives them rights to derivative works. The first depends on what exactly they said when they invited the Elves to Valinor; the second on a lot of complicated factors. I’m personally inclined to believe that the Valar have rights to the Light and Fëanor most of the rights to the Silmarili, but that the Valar have a legitimate partial claim.
What the partial claim entails is difficult to pin down especially when the surrounding culture has little concept of property rights. Probably something like the right to nondestructively borrow them or something. It doesn’t matter much with the Silmarili in Melkor’s crown at the time. The point is that Fëanor was more possessive of them than he had a right to be.