MPM Vs. Miles only - availability?

Just been looking at a certain date in Nov.
If I select miles plus money for a UC seat, there is no availability.
If I select miles only, it comes up with 90,000 miles + 350 or something in taxes.

How comes I have the option of using miles only but not paying cash for that seat? I would have thought the cash would be better for VA than a mileage redemption.

[Darren Wheeler]

Strange as it sounds, a full reward seat is better for Virgin as it reduces their liability for the future. Seats will always sell, but reward seats are always in short supply.