Why is this a surprise? Almost all rental car companies mark up on refueling charges. Every time that I have rented from Sixt, the paperwork/contract also lists the fuel prices.
Would not consider this unethical as you, the renter, agreed to refill the car and understood that they would charge you if you did not.

You'll find a similar experience if you decide not to refuel with other rental companies. My colleague forgot to refill a rental car once (almost completely empty) and Avis billed him for $120 (at ~$10/gallon).