This may all come to a head at some stage, with the EU consumer people.
If you choose to go to a restaurant and skip the starter of a fixed price meal, can they force you to eat it?
A lower level court in Germany has already said not, in the case of Lufthansa, but the case has not been appealed
AFAIK, as the higher courts can make a binding decision.
If the airlines do push it, I'd expect some reaction.