Cross-border rentals in Europe are different than between US states because they're separate countries with their own import laws. It's not like a NC car in Atlanta, for instance, where nobody cares as long as the vehicle is on rent most of the time.

Hertz-transfer.eu has a list of cars that need to be repatriated. FT members in the Hertz forum have said using it gives the regular rate without the usual separate drop fee.