Missed our going flight and now we are forfeiting our RETURN flight too!

We had our dates wrong when we booked this flight. My colleague asked me to book a flight for her tomorrow - return flight to be on Sunday. We didnt check the date and made the booking for today, 27 August instead of 28 of August. We realised only our mistake later and phoned the call center immediately. The gentlemen informed us that we have forfeited BOTH flights and that we have to make a new booking including the RETURN flight? I can understand that we have to make a new booking for our flight that we missed - but why the hell are we forfeiting our return flight too? Absolutely shocking. Flight number ref WPSVUK