She didnt. Much like Tolkein did, she created a world that doesnt exist (though not on the level he did), But the good guys, and the bad guys have the same arsenal.
I mean..Is it a little more compatable, if the Witches are bad, and the good comes from somewhere else, ala Lewis' White Witch, sure.....
But IMHO, to be procedding in this direction gives the Rowling books the heady scent of official disaproval, and by extension detracts from the message you want to send to those very people reading the books.
It would have been far more useflul to say, "They are Kids books..no more no less, and not a problem as long as youngsters are provided with sound moral instruction...'
That's the problem. In my humble opinion the books undermine sound moral instruction (at best) and in some cases actually reinforce immoral lessons.
i don't know how i can make my point any plainer, and you obviously disagree, but i see the choice of witches as protagonists as a value judgment in and of itself. It cuts against the stereotype of witches and makes them more acceptable, harmless and even a force for good (how confusing is that?). your cowboy analogy is inapposite as cowboys have no connotation as good or evil in the first place. witches do.