Yup - sucks, but true. You can argue, as did I, and you'll win the verbal argument, however, they'll retort with "that's what we've been told to do" and not issue a registration until the fee is paid. You'll end-up paying $80.00 more per year than for a non-pickup vehicle.
The short story is that a pickup is a vehicle with an open bed (without a permanently attached cover). A commercial vehicle is one that is "used or maintained for the transportation of persons for hire, compensation, or profit or designed, used, or maintained primarily for the transportation of property". Here's the rub - in the commercial vehicle section of the vehicle code, it states simply "weight fees for pickups trucks are calculated here", but there is nothing that states that a fee is charged. It's a hole, but it's not big enough.
A couple CA representatives have introduced legislation (AB 28 - see link), but it always dies "on the floor". Considering the financial situation in CA, I doubt that this will be reintroduced any time soon.
Maybe some class-action suit or something in the future, but until then, we're stuck.
There is a compilation of information regarding this in my library. Pay attention to the weight limits, as the ST is close to 4,000 pounds and you may be able to remove some things for the weighing to get it under 4,000 (rear seats, spare tire, tailgate, etc.). Be creative, as you only have to get it weighed once, and it'll save you $56.00 per year.