And if someone hands you the keys to a top of the range BMW you'd just assume it's your right to keep it and not worry about the consequences?
If some random person on the street handed me keys to a random car that may or may not be theirs, then yes, I would be at fault for simply taking them at their word.
However, if the car showed up in my driveway, sent directly from BMW, with all the documentation saying it was in fact my car, then that is BMW's fault, not mine.
Was it a dumb move on the part of the guys that used the item? Certainly, but only because their use of the item occurs in a world in which Blizzard holds absolute power, can keep track of their every move, and can distribute punishments and rewards as they see fit regardless of any actual circumstances.