Why the 5.4 shadow priest nerf?

In 5.4, shadowform is losing its 15% damage reduction, which we've had since vanilla.

I am honestly puzzled by this change. Shadow priests are already sitting ducks in PvP, and this change will make it even more severe. Not only that, but this significantly hurts us in PvE as well.

I understand that these are early PTR notes, but it is extremely discouraging due to the fact that every shadow priest nerf on the PTR has made it to live in MoP.