From FT, not sure if his correct but laid it out nicely
I disagree
If WR cancels the tickets, then UA doesnt have to transport you. If I buy a ticket from a travel agent and for whatever reason the TA cancels, then UA doesnt have to transport me.
Its like UA getting a order to transport you. When the person who placed the order cancels, they will not transport you. They dont have to, as you dont have a valid Voucher for the flight. UA will say, sorry, but we didnt cancel it