A mage who does not receive the teachings of the Circle and who does not have the words of Andraste in her heart is an apostate, and a danger to us all. Without the guidance of the holy Chantry, a mage may foolishly dabble in the darker arts—blood magic, or demon summoning, thus becoming maleficarum. And a mage's mind will ever be a doorway to spirits of the Fade; without proper instruction, this doorway remains open and unsecured. If a demon should come through this doorway and possess a mage, an abomination is created. Abominations know only madness. They cannot be reasoned with and will slaughter man, woman and child without thought. Whole cities have fallen to these creatures. Thousands have died at their hands.
The Chantry and her templars have a duty to ensure that this does not happen.
If I knew a better way to deal with magic, I would seize upon it immediately. You say we should let the mages guard themselves. I tell you that this is no solution. Look at the Tevinter Imperium. Their magisters do not know restraint. Without Chantry oversight the magisters abuse their power. Those without magic are trampled underfoot and forced to serve. Slaves are slaughtered by the hundreds to feed the magisters' hunger for power. Even some mages are not spared, for in mages as in all humans, there exists a spectrum—on one end, the very powerful, on the other, those that can barely light a candle. The Empire cares only for the strongest, and those who do not compare favorably are thrown to the wolves.
Imagine your children growing up in such a world. If a mage asked it of you, you would have to give him your daughter, not knowing what his plans for her might be. You could not resist him, and neither could she. Without our templars and without the Circle, the common man would have no defense against magic. We must deny the mages certain freedoms for the common good. I wish there was another way. I tell the apprentices this is a test of their faith, that it is the will of the Maker. Many understand that we do what we do for their own good.