The rear brakes on a FWD car don't do much. The car is front heavy so under heavy braking most of the car's weight is going to be on the front axle.
Honda did not use 2 piece rotors or a vented design on the rear to save money since it wasn't necessary from a performance perspective.
Are there benefits to a 2 piece rotor? Sure. Just not worth it for Honda to put them on the car and make the average Type R consumer pay for the extra hardware.