Other mistake: The second bus in the scene before the crop duster has no license plate.
Other mistake: When Cary Grant steps down from the bus from Chicago prior to the crop-duster scene, naturally the bus is heading away from his departure point. Later, when he hijacks the car to effect his getaway, he makes a U-turn so that, instead of returning to Chicago, he is heading in the opposite direction.
Answer: More than likely, they felt that Roger would be dead and they would not be found out. The fact that he survives their DUI plot and returns to the house with the police only serves to makes him look more suspicious and guilty. It's to move the plot along, nothing more.
ChiChi