Indeed, it should be the actual use that governs this. If you use it as a passenger vehicle, it's a passenger vehicle. If you use it on a farm then it's a farm vehicle.

There's already special registration for farm vehicles in a lot of states. The taxes are lower and that's one of the few cases where you can use pink diesel, which is also taxed differently.

Trucks used as passenger vehicles are already more expensive to register and run than farm vehicles.

It’s pretty difficult to require a manufacturer to design a car today based on how someone will use it in the future.

Huh? To drive around?

A manufacturer does not know, at manufacture time, whether you will put hay bales or grandmas furniture in your truck bed.

Sure, but no matter what, there might be a kid of the road, even on the farm.