"A spokesperson for the airline, who did not give his name, told CBC News that the airline is changing its policy because the weather is so unpredictable during March break and Christmas."
Well if that's the reason, then why not apply the new "policy" during those periods?
We all know that the real reason is that AC doesn't want to have to bother with caring for a UM during connections. That is one parent (or guardian) drops the UM off, another picks the UM up at the other end, and AC need only concern them with getting them from security to the plane and then from the plane to baggage.