I assume your question is based on the theory that if UA doesn't have to pay the tax to IRS (because the passenger doesn't fly), UA should refund the tax to the consumer.

Unfortunately for the consumer, there is a litany of cases, Contract of Carriage provisions and federal laws that allow UA to keep the taxes.

Consumers have frequently filed suit against airlines, seeking to obtain refunds of the excise tax, using the argument in my first paragraph. To date, no one has been successful.

One would think it's a slam-dunk to get a refund, because Section 6415(c) of the Internal Revenue Code provides that airlines required by section 4261 to collect the air transportation excise tax shall, upon proper application, refund any over-collection of the tax that they have made.

However, there have been some court cases that hold that even though cash might not change hands, the airline does in effect refund the ticket price (including taxes) to the consumer that doesn't fly and then takes some or all of that refund back in the form of a change penalty.

In the case of a nonrefundable ticket, the airline is deemed (for tax refund purposes) to be refunding the total ticket price (including taxes) and then charging a fee equal to the ticket price to cancel out the refund.

The bottom line is that although you pay an amount that is represented to be a tax when you buy your nonrefundable ticket, your contract with the airline provides that you don't get any of that amount back if you choose not to fly.

A few of the many references on this topic:

UA's Contract of Carriage, which - in Rule 270 - doesn't allow a waiver of the nonrefundable provisions in the event you request a refund, except in the involuntary refund situations described elsewhere in the CofC.

49 U.S.C.App. sec. 1381(b), which allows an airline to engage in certain ticketing practices so long as it complies with federal regulations.

http://www.ca7.uscourts.gov/op3.fwx [Search for case 92 (year) 2062 (number)], in which a consumer lost her suit against AA, wherein she claimed that AA was obligated to return the taxes she paid for a flight she didn't take.

http://www.ca7.uscourts.gov/op3.fwx [Search for case 96 (year) 3094 (number)], in which the consumer lost where the court deemed that UA had refunded the tax and had offset the refund with a penalty.

http://www.ca7.uscourts.gov/op3.fwx [Search for case 96 (year) 2736 (number)], in which the court held that it's just about impossible for a consumer to force an airline (Southwest, in this case) to refund the excise tax.