And it should stay that way, people who earned the weapon by doing Naxx 40 should be able to say they actually did the content to get it (even if they were lvl 70, the place was still difficult unless you were geared out completely) rather than some nobody who never even heard of Naxx 40 walk up to the black market, and buy it for some gold.
I can understand why T3 is being brought back, it was a great set that very few people got to complete entirely, my brother still has only 3-4 pieces of his T3 sitting in the bank, and I am sure he would like to finally finish the set when he was unable to.
But something like the Corrupted Ashbringer is something that only people from the time it was relevant know its value. The fact that it was to be a legendary gives it alot more weight as an item compared to some random epic that dropped in Naxx 40. Not to mention the heavy retcon of the weapons history and its repurposing in WotLK.
The weapon shouldn't be allowed, because it is not supposed to be in player hands in lore to begin with, Blizz just allowed the players who got it to keep it for the sake of being nice. Putting it on the Ah does nothing but provide fan service for wannabes of a forgotten era of the game.