Thanks for the clarification. Since this is the final leg of the journey, other than being a noshow and AC possibly trying to track you down at the departure gate, or annotating you PNR file (highly unlikely), you will be long gone from the airport by the time they notice you missing from the ATL flight. (Just don't try and claim Aeroplan points on the YYZ-ATL segment.) I don't think Cdn Immigration has any concern as long as you are qualified to enter the country. They may ask about your onward arrangements if you are not a citizen or Cdn passport holder.

The real issue is if you tell AC in YYZ that you don't intend to continue on to ATL, you are in violation of your ticket/fare conditions and can be charged the fare difference. This is a clear case of "hidden city" because you are using the cheaper fare to ATL, but only travelling as far as YYZ. It would be up to the agent you speak with to "penalize" you. They could insist you board the plane because you are technically in transit. They could bring the RCMP around to make sure you do go through the transborder preclearance, if you are not a Canadian citizen. However, I think it is more an ethical decision on your part. If you noshow, I don't suspect anything will happen.