Something has to be a consideration, whether it's status, fare paid, who is using Avios or whatever. If there is a 'plane with 14 F seats waiting to fly to LHR and 15 people have F tickets for that service then somebody has to draw the short straw. And if 14 people rock up to the airport with boarding passes and the last to show up hasn't checked in and there are no seats left, then that last person might well find they are the one! That is all anyone has said. No one is defending the policy or suggesting this is how things should be, but on the rare occasions someone has to be booted out of F a failure to reserve a seat or check in, or even just showing up last may be the factor that gets used.