A couple of things to consider when trying to rationalize a free mage Hawk:
in Act 1. First, Hawk lives in low town. The guards have an extremely light presence there and the people that live in low town have a very "Mind your own business" way of living. Second, the first year of living in Kirkwall, Hawk has the Merc band or Smugglers silencing people (by bribe or threat) that might raise a stink over the fireball throwing mage in Low Town. Not surprising no one would go running to the guards much less the Templars.
In Act 2, Hawk is one of the wealthiest people in the city with ties to mercs/smugglers, he/she has friends in the city guard, and the Viscount owes him/her a tremendous debt for the safe return of his son. (granted the kid was not in any danger until the Viscount tried to "rescue" him, but still...) All and all he/she has been a boon to the city. So again we have plenty of reason for people to ignore the occasional fire ball or lightning bolt.
In Act 3, he's a full blown hero. Every noble owes their lives to this mage. What are you suppose to say? "Thanks for saving us, now kindly go to prison." Me thinks nooo. By this point Hawk is effectively untouchable. Mage or not.
Of course that's the heroic Hawk version of justifying it. There is always the not so nice version. You can say he/she has always been a blood mage using blood magic to warp peoples perception of him/her. Another reason no one ever says boo about it.