Q. So you informed us to be cautious of "throwaway ticketing" (not using the final ticket of your trip) because airlines will "do every thing they can to penalize travelers when they do this". My question is: Why? What negative difference does this make to the airline? The plane is lighter for the leg you miss, they don't have to give you a drink or, if you should be so lucky, a snack/meal. They don't have to load or unload your bags, and yet they are getting paid as if they had provided these services. So why do they care if you skip the last leg of your trip? They probably double booked your seat anyway!
A. Because one-way tickets are often more expensive than round-trip tickets and they don't want to lose revenue as a result of people buying up round-trip's and using them just one-way. That's particularly true of fares requiring a Saturday night stay, which are often cheaper round-trip than some one-way fares. Business travelers are usually willing and able to pay more for fares, and they often use one-way tickets.