They don't. Every parent I know that has tried this has had a shitshow if a problem unless its carrier metal. Its overall a very poorly implemented concept. Sounds great on the surface, often times a nightmare. Lap Infant fare just doesnt seem to carry through, likely due to the way each carrier records it. I'm trying to set some realistic expectations. I have a 19mo old that we've logged damn near 90k miles with. Its either stick with booking via the operating carrier or buy a full seat. BA is on the banned list until 2yr mark because they are the worst to work with, including with a paid seat.