I'm not arguing this point but it makes me think: what kind of 'knowledge' do you need to create a zombie? There's no stunt requirements for that kind of thing.
-Should a practitioner of divination of some kind of schooling in Astrology, geomancy, astronomy?
-Does someone doing fire magic need to have a physics stunt so that they know how to create fire from air?
-Do you need a veterinary degree to turn someone into a dog?
I just wonder why healing has this, seemingly, arbitrary requirement when most other rituals have the knowledge built in.
It's not arbitrary--not as Jim has described how healing works. You've got to
picture in your mind exactly what you're doing. Without some kind of advanced medical training, you're simply not going to be able to direct your magic to do what you want it to. Healing isn't as simple as "close that laceration! Magic!" Without the training (or an intervening sponsor like Summer), you're guaranteed to mess up your healing. And then you're a Lawbreaker.
Personally, I don't require the Doctor stunt for a PC to heal, but I require an appropriate High Concept or other aspects to justify the knowlege they've got of human anatomny. A CPR class at the YMCA won't cut it.
Making zombies is a matter of calling up ectoplasm around a body and animating it like a puppet. Vastly different than repairing living tissue without negative side effects.
If a diviner uses astrology/geomancy/astronomy in their divination? Yeah, they need to know that kind of stuff, whether through an appropriately high Scholarship skill, aspects, etc. Let's also pause to reflect how long it takes to gain a working level of knowledge in these subjects compared to the time it takes to become a doctor.
For the fire thing, obviously there's hyperbole going on here; the whole "years of training in the mystic arts" covers that. And wizards do, to some degree, learn physics to help them cover it. Harry has mentioned many times that magic still obeys physics--implying he has gained a working knowlege of it as it applies to his magic. Why the difference between this and needing additional justification for healing? Because the physics behind fire, and the mental construct needed for that, is much simpler than the complexity of biology.
If you wanted to turn someone into a dog, you don't need a vet degree--but unless you had one, the person-turned-dog would die shortly from having organs and systems that don't work properly. As is mentioned somehwere in YS in the transformation section, I believe.