I have seen that often written, that One Way rentals within Florida, from Florida to Georgia, within California and from California to certain states (Nevada, and IIRC Oregon, Washington and maybe others) are free. Not only for Hertz but for other companies as well.

Maybe that shows only when booking through European websites.

PS: I have actually done it in Florida - I booked a one way from Miami to Jacksonville at no extra cost on a European booking website. But there was a problem: The car rental company in Miamia Airport (not Hertz!) had lost my confirmed rental. I had to discuss this with the manager and take out a new rental contract (took forever) and was promised I would not have to pay anything extra when dropping the car in Jacksonville - the contract print out didn't state it "due to computer issues" but some handwriting was added). Of course there was a huge fuss made in Jacksonville when I turned up with the car and I was lucky I had arrived way early for my flight. It took a lot if time. After returning home I brought everyting up with the booking company and, in the end, all the extra money had to pay (new rental, maybe a one way fee, I don't recall) was returned - but there was a small loss due to fluctuating exchange rates.