The reason for the huge one way fees in Europe is that citizens / registered residents of any European country are prohibited from driving a car with a foreign registration inside their country.
Otherwise it would've been an inviting loophole to avoid ridiculously high car taxes in Scandinavian countries, Denmark, and the Netherlands.

This means that once you drop off a car in another country, Hertz will have to either wait for another foreign customer or ship the car back.

That said, sometimes they have excess of cars from other countries that must be returned home. You can try to negotiate with the agent at the time of pick-up to be mutually beneficial.

There is a Hertz site hertz-transfer.eu where such cars a listed live. However, it is for internal Hertz use only since a few years ago when they had stopped accepting customer reservations with info from that site.