It's actually much simpler than you think. The European Commission (DG-TREN) has expressed concern than airline contracts are too rigid and provide little protection to consumers when an airline ceases service.
Because they don't want the EC to impose obligations on other airlines to accept stranded passengers, either FOC or at an arbitrary level, the airline organisations have counselled their members to offer rescue fares to cover these situations.
As long as these fares remain 'reasonable' the EC will probably stay out of it. But if they detect too much greed (I.e. the law of supply and demand) that will change..
Isn't it nice that the Commission feels it appropriate to interfere with basic economic laws.