Earlier quoted context omitted.
Because the courts have found it is acceptable? (The court here references a decision of the highest German court saying so, and rejected the claim only because Lufthansa wasn't transparent enough in how they calculated the difference)
Let me clarify: I have no idea how a legislator or court would think this is reasonable either; I'm equally interested in a courts justification to how an airline can force a customer to use both legs and this NOT being equivalent to e.g. McDonalds requiring people to finish both the burger+fries if the meal is cheaper than just the soda+burger.
The case in front of the higher court was against airlines that cancelled tickets (and denied transportation) when a leg wasn't taken, and the court decided that this was too too unfair against customers that had legit reasons to not make use of the leg and offered the requirement to pay the difference as a compromise that still discourages passengers that plan this intentionally.