Morality/ethics aside, on all of my ticket purchases I have not seen anything where I can be charged for not using a segment.
I assume this is the stuff like "NONREF-0VALUAFTDPT-CHGFEE -REFUNDABLE-CXLFEE-CHGFEE"...
Section 9, Contract terms:
http://airconsumer.dot.gov/publicati...s.htm#contract
There are additional notice requirements for contract terms that affect your air fare. Airlines must provide a conspicuous written notice on or with the ticket concerning any "incorporated" contract terms that restrict refunds, impose monetary penalties, or permit the airline to raise the price after you've bought the ticket.
If an airline incorporates contract terms by reference and fails to provide the required notice about a particular rule, the passenger will not be bound by that rule.