It is not fraudulent. You paid for a 1-way ticket and got a round-trip ticket. You were given more than you paid for by the vendor, essentially. It's like if I order a book and they send me the book and another book with it for free. That is not fraud.
Also, the New York Times ethicist declared throwaway of flight segments to be okay.
Purchasing something doesn’t mean you’re obligated to consume it in totality. You can use whatever portion of the purchase you choose. If you buy a loaf of bread, you don’t have to eat every slice.
http://www.nytimes.com/2014/09/14/ma...for-speed.html