As Raffles says, the cards that are marked are people who have delayed the return leg. (Although paradoxically I would have thought they are the ones who are most likely to fly it having changed it to a convenient time?)

As to enforcement: all BA need to do is 1. No TPs for that particular trip (good luck sorting that out 'in court' or 2. No more avios or TPs for you ever (again, the problem becomes the passenger's to try and beg forgiveness). If you've broken your leg and can't fly I'm sure BA will be happy to reverse their decision on production of medical evidence.