I love how the use of real world laws about counterfeiting stuff gets turned to excuse not adding content for player use.
Oh noes, a unique NPC's item might have lookalikes out there. There must be a law. Unless you're telling me that heroes and villains are running to the patent office and/or getting their equipment's appearance trademarked, this is a laughable excuse. And even still, half of this game is centered on people playing as CRIMINALS who regularly break and even flaunt the laws of the land because they can. Like, "I will challenge Longbow and the Freedom Phalanx themselves, but I better not have a copy of this unique looking artifact because that would be against the law."
NPCs having unique items can be represented in theirs granting special abilities that copies won't have. The Ghost Slaying Axe is a good example of this. The original developers on Live never wanted to proliferate its model to the Battle Axe power set because they wanted the prestige power to remain special. It always would have been special because it causes an additional damage effect against undead that copies in Battle Axe power set, and the fact that it was finally added here on HC proves it. No one laments that their prestige power Ghost Slaying Axe isn't special anymore because people can use the model.