This is not the airport/segment tax. It is the 9/11 Security Fee.
http://www.tsa.gov/research/fees/passenger_fee.shtm
$2.50 per segment, $5.00 max in each direction.
I have a recent award booking with three segments - fee was $7.50.
I do not expect UA or any carrier to pay government mandated fees/taxes on my behalf when I travel on an award ticket when the taxes/fees are not a function of the price of travel.