Befree
I agree with what you are saying, however also remember that FR will also rely on revenues made whilst fleecing you either at the airport or onboard the aircraft. As such, if you have booked seats on a number of different flights for the cheapest possible fare, then FR will be losing out through no-shows (assuming it doesn't overbook???).
Unlike the likes of BA who've basically made all they revenue they are likely to out of you before you check-in; FR will still be budgeting a fair percentage of its target revenue per passenger post-booking, which it obviously fails to make if you don't turn up.