As for why tank healing is so out of whack, I could only hazard a guess. What I would imagine is that they have internal data that shows that failing casual content is a significant driver of sub cancels. Throwing some extra healing at tanks is a way to add a bit of a failsafe to that. If the tank is good and the healer is bad, they can muddle through. If the tank is mid and the healer is mid, the extra tank healing probably helps. If the tank is good and the healer is good, the healer dies of boredom but that healer probably ditches healing instead of cancelling. Or maybe ditches dungeon queues.

Which is bad, but is a less bad outcome than a cancelled sub. This also assumes that the bored healers actually quit, or even exist as a significant plurality. For all we know, their internal data shows that the shb healer changes have led to a positive uptick in the percentage of the games population playing healers, which could be why complaints have fallen on deaf-ish ears.