I agree that healers should know this... but I can imagine seeing the need for heals before the next pull, and assuming the tank will WAIT until everyone tops off before pulling. There may be some shared responsibility here.
I'm guessing this is WAI, and it seems like a challanging mechanic, not sure square should just nerf it.
***edit I don't expect healers to magically know the second they get regen, I do expect them to figure it out after 1 or 2 pulls, and then stop doing it.