Because everyone who isn't strong enough to control their magic would want to go to the circle. You have metal detectors at court houses not because everyone will bring a weapon, but because everyone has the potential of bringing a weapon, and that's why the Mage's are kept in the circle.
Ahaha, so if I was a confident and emotionally balanced mage in Thedas I should be held accountable for the 'possibility' of some unforeseen disaster by pathetic mages who don't have the sense to say "no" to a demon offer? I'll take Apostate for 200 Alex. Hello Morrigan, Bethany, and Solas, oh you want to help stop the Blight, oh you want to help survive and protect your family from the Blight, oh you want to help stop the Breach? Thank you for your magical services with no Circle training or forced lifestyle.
Then come up with a working system for those mages rather than punishing an entire people and also for those mages out there who commit sinister acts. If a mage has shown the community of his/her area that they can be trusted and have no record of any crimes then they should be able to lead their own lives and practice their talents in peace. Everyone is tempted with offers, pleasures, and opportunity on a daily basis, that doesn't mean they succumb to those things.