Well because I would have sworn an oath not to let anyone touch it, and then what would have happened when someone came to take it away and put it in the case? What kind of crazy thing would I have been tempted to do, since I just swore no one would touch it but me?
I mean I'd have to be forced to cut the poor kid's arm off or something, then, or maybe at least speak very loudly at him and snatch it back like a crazy person... then they'd probably leave and I'd have to get a new shop keep. Those are hard to come by, you know. Nerdanel would give me a stern look.
Worse if it was one of her spawn that'd gone touching it, and if I slapped him it'd be the Maedhros thing all over again with the 'now my son is a partial amputee it's your fault why weren't you paying attention'.