While it's easy to understand why some kinds of magic would be forbidden, I can't help but think that The way they control which spells should you learn are far more harmful than simply allowing the use of Forbidden magic in a more controlable manner. I mean the main problem is not the forbidden magic itself, but the people who might try to use it. And while it's easy to claim that absolute power corrupts absolutely. That's not always the case, some people can indeed have access to some sort of power(in our world it can only be political though) and manage to not be corrupt by it. I've personally met a Mayor who used money out of his pocket to supplement the capital available to his town in order to promote his city and ensure their schools would have better education. And by personally, I mean literally, the guy is such a chill guy that he's always doing rounds around the city and talking with pretty much anyone he meets along the way.
Edit. Please delete this post, I've confused the edit box with the post box right above.