If I recall correctly, the seats in the back of the Brat were there simply to allow Subaru to import it as a passenger car rather than a small truck, saving a whole lot of taxes (or possibly having something to do with emissions or mileage standards). They were simple to remove and almost everybody did.
Yeah, you are correct. The intent from the Government was much more sinister though:
The US imposed all sorts of wierd things for foreign products over domestic ones. The Corvette, for instance, was allowed to have foglights that operated without the headlights. Imports had to have the foglights operate only when the lights were on. The little things.
So yes, in an effort to block foreign trucks in the US, they increased the tariffs on foreign trucks. The seats were there to get around a loophole. But they are neat, and other countries enjoyed them.
Government has been doing their modern tricks for a long time now.