I don't remember where I was, it was in the US, not even sure it was Hertz, but did notice the airport had imposed a car wash fee at their return facility. I thought that was strange, and apparently after a bit of googling, they farmed the car wash out to a central facility and the county or whoever gets a cut of each $2.50 fee they impose on the rental agencies, and require them to add the fee to the rentals like all the other taxes and fees. It was just a pass along in that case. Not saying it's the same in your situation, but it could be something they have no control over.