It's an interesting point of law. You buy a flexible ticket of any kind, you've flown the outbound (historic pricing) using public resources (their website, GDS) you see there are seats available, but the carrier says no, there are not. They control this situation, you are the weaker party. You're then forced to purchase a new ticket, in the same compartment, for the flights you wanted on which the carrier says there are no seats. (Did I understand the facts correctly)?

Not only is it unfair, it seems unlawful, breaking the basic tenets of good faith in contract law that make up the Civil system. That's without looking at Unfair Terms statute and probably tonnes of other stuff.
[MENTION=168067]athome[/MENTION] you should take a look at the sheer volume of cases Lufthansa loses in their home court of Köln. Google translate can help. They are super dodgy.