[ChrisH]

You can try showing up to the particular airport and telling them that you did take the entire trip and don't understand how or why it would show you as a "no show". You may at least get on a standby list. Don't cause scene, but if you get the right agent, they may be willing to help. It does happen sometimes that people's reservations are cancelled even when they did take the flight because their boarding pass doesn't scan properly.

As per the airlines do this. They basically all do it. It is to avoid people purchasing a two way ticket, because it is cheaper, but only wanting to take the return portion or just fly certain segments of the tickets.