Surely the decision/policy on this issue should be decided by the immigration department. Is it legal for a transport company to impose such a restriction? On what grounds? Surely it is your responsibility to arrive for the return trip with a valid passport to enter the country of destination. It must be valid for first departure. Your on a long weekend trip, returning in 4 days on a passport with still 4 months valid, and they refuse you travel. Has anyone tested the legality of this? Does one company do it because the others do? If you travel by car/train the responsibility is all yours.
James: if the trip is within Schengen then the passport is only necessary for an ID at check-in. There is no immigration issue. Is a photo driving licence an acceptable photo ID? It is where I live.