The main argument against in this case seems to have been that it's a surprising clause in the terms (which would invalidate it), with the counter-argument being that it's well known that these price differences exist, and thus shouldn't be too surprising to a consumer that the airlines added rules against their "clever trick", because otherwise the pricing wouldn't make sense. Without a rule forbidding it, it's contractual freedom.

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.