: I'm speculating wildly of course. We'd have to define what powers the Silmaril gave its holder, which I think might be quite tricky. But Thingol was apparently impressed by the fact that it was so, and considered his demand on Beren fulfilled. Of course he was the victim of his own unfortunate wording. But maybe the power of the One Ring was controlled by equally unfornately worded spells. Did Sauron think of adding a sub-clause to the spell stating that the finger had to attached to the bearer.

I dont know if any of the Silmaril's power could be used, but I think that Thingol was more impressed by the courage and audacity shown by Beren,, not to mention the UNDENIABLE love shown between Beren and Luthien.:)
Also, technically, Thingols demand was fulfilled ("Bring to me in your hand a Silmarilfrom Morgoths crown...").