Lot of misinformation here.The fact that Thunderfury can be used by any class that can wield a sword does not mean you go looting bindings to non-Hunter/Rogue/Warrior/Pally.The bug itself (which is apparently confirmed not a bug), is that you have an actual Warrior/Hunter/Rogue/Pally do the quest chain, and then once you kill Thunderan, loot the next quest item: Dormant Wind Kissed Blade to some other class, like a Warlock or Mage.Because the quest item starts a quest, it does not matter that they were not the original holders of the bindings themselves, and are free to complete the quest and receive Thunderfury.Typically, those players you see with it, paid an astronomical amount of gold (or $) to get their guilds to give up a TF on one of their warriors.
The class requirements in this case are guidelines, not actual rules. As you don't need to equip these, the quest only requires you to have them in your bag. TF has no class requirement.