We don't. Perhaps we can't. In the South, they control magic by keeping mages prisoners in dingy circles under control of the templars. The Qunari are even more repressive to their mages. But in the Imperium, mages rule and are permitted to use their magic to seek influence and power. A number of forms of magic are officially banned, but so long as you're from a good enough family, don't fall out of favor and aren't too blatant about it, there's nothing to stop you.