Magic is chock full of symbolism and intent. Each spell and magic item is crafted to suit the expectations of the creator. The original creator of the armor clearly intended it to be a source of bonding between the rider and his draconic ally. Scales taken through an aggressive or hostile means would have worked against that intent.
I would assume that even the most friendly of dragons would likely take care of their offcast scales, as such would be valuable as reagents, or could still hold some arcane tie to the dragon itself. Therefore, to receive such scales would require a measure of trust, which would carry over into the creation of the magic item.
I would assume that even the most friendly of dragons would likely take care of their offcast scales, as such would be valuable as reagents, or could still hold some arcane tie to the dragon itself. Therefore, to receive such scales would require a measure of trust, which would carry over into the creation of the magic item.