Apparently you don't understand how airlines work. All airlines have some sort of non-refundable ticket, which is usually the cheapest ticket. You can pay more and get refundable tickets next time if you have a problem with this.
Additionally, the woman who answered your call would not have had the ability to simply claim two more seats on the plane that easily. If you weren't asking for your money back, then she still would not be able to sell those seats because there's a liability there - what if you suddenly decided that you did want to take that flight back? If they'd already re-sold the seats, then you would still probably demand to get them back since you had still paid for them and hadn't gotten a refund.
But most importantly, are you telling me that you weren't asking for your $400 back, but refused to pay $100 to change it to a one-way ticket? It seems to me that if you're able to change it to a one-way ticket from a round trip by simply paying a fee, that there should be some sort of refund for at least the second part of the trip. Maybe I'm wrong - I don't know exactly what you two discussed.
But the bottom line is that you would have this problem with any airline. Next time, if you really aren't sure what your plans are going to be, pay the extra money to buy a refundable ticket.