The ultrabasic customers cannot pay for cabin baggage. They can bring their small cabin baggage. If they have a large one, it can only go into the cargo hold.
Part of it is also that it's a choice. Everyone who buys this fare knows this. This is not a surprise. This is something that someone deliberately chooses. We had 1.2 million Canadians who bought an ultrabasic fare, did not bring a bag for the overhead bin and also did not check a bag. That's a real saving.
If they do show up with a bag, it has to go into the cargo hold.