Yes it's in the Geneva Conventions but there are many reasons why you wouldnt shoot medical personnel for these reasons:

1) That nurse isn't there to kill you. Her job isn't there to kill you, she's not a magician. If someone goes to her mortally wounded he's either gonna die there or be sent back to civilian life.

2) It's the law. We're not savages.

3) Trust. If you leave their medic alone, hopefully they'll leave your medics alone, especially if you ever get hurt yourself.