'Hidden city' travel: why airlines are cracking down on a discount trick

There have been many submissions of this story, some with significant discussion:

    Lufthansa Sues Hidden City Ticketholder
    for Throwing His Ticket (cnn.com)
    111 comments[0]

    Lufthansa Sues Passenger Who Missed His
    Flight – 'Hidden City' Trick (slashdot.org)
    10 comments[1]

    An airline is suing a customer who skipped
    a leg of his flight to save money (vox.com)
    6 comments[2]

    Lufthansa sues customer for skipping flight
    (godsavethepoints.com)
    2 comments[3]

    'Hidden city' travel: airlines are cracking
    down on a discount trick (theguardian.com)
    No comments[4]

    Airline sues passenger for not taking a
    connection (ctvnews.ca)
    No comments[5]

[0] https://news.ycombinator.com/item?id=19144006

[1] https://news.ycombinator.com/item?id=19153457

[2] https://news.ycombinator.com/item?id=19159914

[3] https://news.ycombinator.com/item?id=19126675

[4] https://news.ycombinator.com/item?id=19182808

[5] https://news.ycombinator.com/item?id=19156064

I may be misunderstanding but the article seems to be saying airlines are claiming against lost revenue from not being able to resell the seats not used by travellers who skip a leg. This makes no sense. They sell the ticket. Whether the customer uses it or not does not cause the airline to lose money.

On a related note I feel like overselling flights should be illegal. You're trying to sell the same thing to two different people, which in another industry would be considered fraud.