Continuity mistake: When Thornhill watches the road from the crop field, the way he holds the crop and how the leaves bend, differ between the front and back shots. Also, his hands disappear and the leaves swap between dry and green.
Continuity mistake: At the field, the first time Thornhill throws himself on the floor there's just a road with gravel. When he stands up there's suddenly hay everywhere.
Continuity mistake: During the crop-duster scene, the sun appears to gyrate around wildly, particularly when Cary Grant runs towards the Magnum Oil truck, forcing it to stop. The sun at this point is low down at this point with the shadow going long to the right (evening). When Cary Grant stands up and the driver tells him to run (after the crop-duster crashes into the truck), the sun has backed up to almost overhead with a very short shadow to the right (mid-day). This scene was apparently shot on the Garces Highway west of Delano, California in the flat Central Valley, a short distance east of Corcoran Road. Garces Highway is almost exactly east-west. All through the crop-duster scene the shadows appear in unexpected orientations from take to take.
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
The bigger plot hole is, if Van Dam really believes Roger is Kaplan, why would he think that Roger would bring the police and go through the trouble of preparing "Mrs. Kaplan" to make the police think he's crazy? If Roger really was a spy, he doesn't need help from the police and would have just disappeared instead of retracing his steps. So if Van Dam anticipated the actions taken by Roger, he must believe at some level that Roger is telling the truth and would have looked deeper into it.