If the fee was disclosed at the time of the reservation/rental, then OP is simply lucky and received a customer gesture. If it was not, whether the fee is legitimate or not, Hertz should have eaten it as it did.
What OP and many others miss is that local airport authorities levy all manner of fees on rental agencies. One of those is the fee for operating a shuttle bus. It is disproportionate because it is a way for the airport authority to generate revenue.
This is no different than "rental concession fees" which are essentially a fee imposed on the rental agency for each rental processed at the airport.
I am not a fan of these, but I am also not sure what the better plan is short of raising landing fees and thus airfares.