nailed it.It comes down to a philosophical point of view. Do you truly change nothing and allow the game to exist in its buggy and easily exploited state? Or do you make minor changes to try to make the players treat the game the way it was back in Classic?
A prime example of that would be the AV changes. Back in the day AV was a several hour/day marathon. Modern players discovered that rushing was the correct strategy for honor farming. Do you adjust AV so that the playerbase is incentivized to play AV like it was in Vanilla? Or do you allow the playerbase to fundamentally change the game with new strategies using original mechanics?
This item is yet another example. Tank threat is the 'bottleneck' in raid dps. Your player's threatcap. Many classes can easily outpace tank threat, especially in AQ/Naxx gear. Tanks need to balance survival and dps. They also don't have the easiest rotation with rage limits. They also require extensive world buffs and consumables to perform alongside their dps. This item trivialized threat and would have allowed tanks to go 'full turtle' and ignore everything except survival. It would have fundamentally shifted how tanks gear, spec, and play at high end content.
Blizzard opted to make a change to keep players playing Classic like they did in Vanilla. That is a philosophical choice. This item wasn't used as a free threat generator in Vanilla. It's philosophy at the end of the day - there is no 'correct' answer. Some will want the game to feel like Vanilla, others will want the game to be Vanilla. Both are not possible.