Airlines can prevent you from boarding if you travel part of a route you booked by alternate means? I'm not generally much of a fan of government intervention in private business, but that should probably be illegal.
I was looking for a flight from Frankfurt, Germany to Toronto, Canada half a year ago. Lufthansa charges around
$1.8k for that.
At the same time, Lufthansa also offers a connecting flight from Delhi, India, to Frankfurt then to Toronto. This one, although much longer, costs $1.2k.
Then I stopped worrying about the world's oil supply.
Similarly, a friend was moving from Amsterdam to NYC, and needed a one-way flight. The direct flight was pretty expensive, but a flight out of Berlin was many-hundreds of euro less... with a connection in Amsterdam (of course).
As you would expect, the airline wouldn't let him just board in Amsterdam. So he packed his things and took a €75 train to Berlin early in the morning, and had his family wish him farewell later that evening back in Amsterdam, during his connection.
It was a pain in the ass, but it saved him a ton of money, at great expense to the airline.
At the same time, Lufthansa also offers a connecting flight from Delhi, India, to Frankfurt then to Toronto. This one, although much longer, costs $1.2k.
Then I stopped worrying about the world's oil supply.