A t+ fare allows for advance seat selection. I think requiring UM to pay T+ fare is reasonable as the UM program requires advance seat selection, which the tango fare does not allow except with a $15 upgrade fee. I think its easier to require a UM to pay the T+ fare than allow them to buy the Tango fare but require the advance seat selection fee.

IMHO, another reason for the n/s requirement is that it would be too complicated to restrict UMs to the most logical connection routing. As an example lets say the UM needs to get from YYC to YHZ. There is the n/s at 12:10 but its $400. Connections are cheaper and there is a 7am departure with connection in YUL for $220 (flights AC160 and AC664), the connection time is 45 minutes. However for $175 the connection can be extended from 45 minutes to 3 hours (Flights AC160 and AC668). The parent is wanting a cheapest ticket will take the longer connection timeframe. This leaves the kid in Air Canada's responsibility for 3 hours.

Air Canada could restrict the connection possibilities, however the passenger looking at AC.com will not understand why they cannot send junior on a valid fare routing. So for Air Canada perspective, its easier to state that UMs can only fly n/s than trying to explain complex criteria for allowing some connections but not others.