"For the same reason you're not striking me down here and now, or, rather, having one of our group do it for you. Other than the obvious fact that whoever cursed me or these three would die, you don't want her to have her magic back. Hermione without her magic isn't much of a threat. Hermione with her magic, and a bone to pick with each one of you, is. I'd argue she has a rather... significant chip on her shoulder."