The impression I got was that the reason that they were referring to what Fringilla was doing as specifically "forbidden" was that she was essentially having these other rogue mages/sorceresses use their entire life force to power the spells they were using (and make super-powered versions of them, I suppose, like the fireballs?). I'm not sure it was the specific schools of magic she was using so much as how she was going about using them.