I've been told (by someone i believe actually does know the truth) that The reason they do it is indeed profiteering. Ryanair have been chargeing for seat selection, but cant get enough people to take it up to make as much as they would like. By making the number of available seats as low as they can on a flight, they make it more likely that families and couples will have to split up into different seats when they get on the flight.
This in turn "encourages" these customers and people who are watching them, to pay extra to select their seats the next time in order to make sure they can sit together. Therefore making Ryanair more money.