Spoke with a couple of people in BWI and all we could think of is what Milesjamie stated above. Customers on a Rapid Reward ticket receive 0 for the cost of their ticket. On a rare occasion, they may offer $50 as additional compensation, to add value to the reroute. I can only assume that the gate agent didn't communicate well enough in stating what the TOTAL value of the voucher would be, not just the ticket value.

While the ticket value of an award ticket is zero, travel credit is still given when redeeming the voucher. This puts you one step closer to earning your next ticket.