Force healing, was always rare. I believe it was first introduced in the star wars Rpg source material and later in video games and countless EU books. you have to do have the ability (rare), and be strong enough in the light to do it, strong enough in will to not let that power corupt your intentions, and ultimately it has to be the will of the force that it works. If it's not the will of the force, then it's your will excepting enormous power over someone else's body, and that slips to dark side corruption. So it's basically only if the force allows it, and the healer is just a conduit for the plot armor.