Visible crew/equipment: Cary Grant is crouching in his bunk bed in the train. The lights of the studio get reflected on his broken sunglasses when he shows them to Eve.
Visible crew/equipment: When Cary Grant and his film-mother enter the elevator at the Plaza this is filmed through a glass-door and you can see the reflection of a crewmember in a white shirt who is crouching below or next to the camera.
Visible crew/equipment: Right before Thornhill gets shot by Eve, he's talking to Vandamm at the table. The camera moves backward. You can see its shadow on Thornhill's arm.
Visible crew/equipment: When Thornhill exits the phone booth 6 stage lights are reflected on the glass door.
Visible crew/equipment: When the crop duster and the truck are about to explode and the truck drivers run away, one can tell it's not Cary Grant watching the havoc but his stunt double because of his rounder face and slightly different hairstyle. Also confirmed by actress Eve Saint-Marie.
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.