In the old days of paper tickets and no computer cross-checks, a Dallas resident could buy (to continue the example of the previous post) Toronto-Dallas-Santiago, show up at DFW for the Santiago flight, get on, return, and get off there with nobody the wiser - or at least nobody able to do much about it. No more. The best you can do today is to throw away the last leg(s) of a trip. That sometimes comes in handy, especially when discount round trip fares are much less than one-way fares, but isn't the same.

By the way, was the OP looking for logic in airline ticket pricing?