There is a huge difference between an international first class award ticket and a $20,000 international first class purchased ticket. The former has significant seat restrictions while the latter is available at any time unless the cabin is sold out. The number of award seats is calculated to be the number of unsold seats (obviously it's a guess since it's a future event but the airlines have plenty of data from past flights and use that to set award availability). Therefore the true value of the seat is only the additional cost of fuel it takes for you and your bags to fly from point A to B which is a tiny fraction of $20,000. The plane is going anyway, the employees are paid the same amount, etc. That is why FF mileage liability on an airline's balance sheet is nowhere near the equivalent of published fares and why it makes no sense to tax them at an equivalent published fare amount.

Look at it this way -- the grocery store bakery department makes enough bread to be available all day long. What's left at the end of the day gets marked down and put in the discount bin for the next day. Those are like the award seats, and the sales tax on day-old bread is less since the purchase price is less. A grocery store typically sells day-old bread at 50% off whereas an airline sells award seats for 99% off because of the massive amount of overhead involved in a scheduled flight.