They are not so much folding seats, as quality of seat and therefore might be folding.
The +2/+3 seats, and treated along the lines of either the +2 seats in the back of a coupe (e.g. Porsche 911). Or the folding/fold flat seats in the back for an SUV/Wagon.
The game treats them as a smaller row than a normal row. Which then feeds into the passenger space/cargo space calculations.