The high rents at YYZ translate into high landing fees charged to airlines. So in this case where AC is essentially price matching a competitive operator at an alternate airport they are absorbing the higher landing fees and either making that up with higher fares somewhere else or taking it out of profits.

As far as the AIF I though they were the same as YTZ raised theirs (they don't call it an AIF but that's in essence what it is) to be the same as YYZ. I suspect the $5 difference in this case is a levy associated with pre-clearance that YTZ flights don't pay as they post-clear.