That may be true, but not essential to the contract violation argument. I'm not taking sides on the issue, and I've dropped a final segment more than once in my life, AND I don't believe anyone is ever going to make a big fuss about it, but I'll reiterate...the airline is offering different fares to different cities. That this fare pricing may seem illogical to us is not relevant. From the perspective of the airline, they are perfectly within their rights to reroute you to get to the final ticketed city C in a way which avoids the traveller's intended destination in B.
From the consumer's perspective I can also understand the tempting argument that no one is getting hurt by only flying half of what was bought, but it is a circumvention of the original fare.