[Roger Thornhill's matchbook carries the initials RoT.]
Eve Kendall: What does the O stand for?
Roger Thornhill: Oh, nothing.
Roger Thornhill: The moment I meet an attractive woman, I have to start pretending I have no desire to make love to her.
Eve Kendall: What makes you think you have to conceal it?
Roger Thornhill: She might find the idea objectionable.
Eve Kendall: Then again, she might not.
Roger Thornhill: I don't like the way Teddy Roosevelt is looking at me.
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.