That sounds exactly right.
Most reward offers and/or coupons for free car rental state in the terms and conditions that the value of a coupon or money off offer, applies to "time and mileage charges" only. Time and mileage charges would basically be the value of the base rate per day of the car that you'll be renting. That means that you still have to pay any car rental taxes or surcharges and fees that are over and above the base rate of the rental car itself.
I went to the United Airlines website and looked up the terms and conditions for the two day free car rental offer forHertz (opens in a new window). The terms and conditions stated, that when redeeming
your United Airlines rewards:
These charges are not included as part of the Award and must be paid for by the renter:
taxes, drop off fees, extra days, applicable charges for excess mileage, any vehicle licensing fees, airport related fees, optional refueling service, optional insurance or damage waiver purchases and any additional driver fees or underage driver fees.