This puzzles and irks me, too. I've talked to central reservations and they've said that if I pick up the car 12 hours later, I don't need to pay the 11% tax.
But, knowing there is a difference between Central and actual (reality), I called the SEA office and they said that there's a tax that the airport charges. They didn't go into whether it was good for 12 hours after your flight arrives or etc and I didn't ask because it didn't seem like he had free time to answer a $10 question. $10 is my difference with that tax, about. And you can't really blame Hertz because it is the airport's fault that there is an additional fee and I think the airport gets the money.

Should I wind up renting 12 hours later than when I arrived, which is possible, I'll ask them to take the tax off and see if they make a big issue out of it or even know about the 12 hour restriction it states on my rental.

I do agree that it is annoying that two of the major seattle locations, airport and downtown (hotel) both charge an extra tax. A small tax, I can understand, especially at the airport, because I appreciate logical traffic flow, nice new terminals, and decent parking garages. But a hotel?! Also, I'd guess that the car rental clerks which work at SEA get some disgruntled customers over the 11% additional fee. As if the original 18% isn't enough?