We don’t have the whole story. Apparently the flight was not oversold but the aircraft had a broken seat. I suspect they did ask for volunteers but none came forward.
I doubt if she actually did have the lowest fare, that would be difficult for a gate agent to determine. More likely she was the one assigned the broken seat and offloading her meant that other pax seat assignments didn’t need to be changed.
She said she was offered a $3,000 voucher but asked for cash. Then she was offered the 10K voucher. Likely UA wanted to expedite the departure.
The actual cost to UA could be almost nothing if she ends up using voucher on half-empty airplanes. If she takes seats others would have paid cash for then UA will have lost revenue opportunities, not cash.