They don't know how healing magic works, as far as she is aware it only heal wounds.
For example, it's possible that it speeds up natural healing temporarily to super speeds.
Or it could regrow lost tissue by feeding on her own energy.
Both methods would have significant benefits and drawbacks depending on situations.
the first example could be deadly if you already had cancer, while the other could possibly give you cancer even if you are in great condition.
The first example might help with getting better when you're sick, while the other won´t do much at all as regrowing flesh won´t remove the sickness.
Just to give a short example...
She can use magic, not using it seens like a waste not something that should be imposed....
I don't rememeber but has she tried to teach someone else to use magic?
I mean since she can use it even though she is in "our" world other people should also be able to do it.
@mahtan remember there was this entire like arc dedicated to people experimenting on her, if they knew she could heal it be worse then before and I'm sure she knows that's, yes Kanji would never ever ever say anything but the small chance of someone seeing or hearing about it is why she didn't do it during normal awake hours. She wants to help kanji but knows she might be seperated again if it becomes known she has that power.
It doesn't matter if it's a fantasy, in the past, or in today's world if people can take advantage of you, they will. Especially governmental powers.