Originally Posted by Often1

The fallacy in this logic is that nobody can force you to fly any or all of an itinerary. The only question is what is the proper fare. If you purchase AAA-BBB-CCC for $300 and the AAA-BBB fare is $400 and you don't board the second segment, you owe the carrier $100. Why you don't board the second segment is irrelevant. You can choose to not board because it's a nice sunny day and you just decide to stay in BBB.

It becomes a fraud (think wire fraud if you booked over the internet or by phone) if you booked the AAA-BBB-CCC with the intent to actuall defraud the carrier out of the $100. That's the OP situation here.

While the carrier can simply zero out & close your FF account and debit your CC for the add/collect of $100, the more likely problem is simply that AAA-BBB goes MX and you are rebooked on AAA-DDD-CCC and wind up in CCC where you didn't want to go or having to belly up to the truth with the carrier at AAA and buy yourself a full Y walkup to BBB.