Every time your enrolled friend flys a fare is calculated based on the "value" of that flight and sent to payroll, that "value" is then added to your taxable income line of your paycheck and you are essentially paying income tax on that flight.

Example...

LAX-ORD-LAX = $400 "value", add that to your taxable income column and pay income tax on it, 10% bracket? That trip will cost you $40. 15%? $60, FO with children? If you pay no income tax, pay nothing!