It may not have anything to do with fear of spying. Drug use is something that a) most people have done, and b) are least willing to admit to. While some of us may shy away from telling people of our sock fetish or whatever, admitting to drug use has real and far-reaching consequences.
So in other words, getting you to admit to drug use would likely be a reasonable representation that they have ascertained "the whole truth" from you.
Also if the operator can't see an alteration in the graphs, he can think that the subject has been "inmune" to the machine and try to break him "manually" like in the linked article or like arockwell's wife. The drugs would be used because, as the interviewer himself said in the linked article: "everybody have done drugs some time", so it's a good guess, even if it's "most people" instead of "everybody".
So in other words, getting you to admit to drug use would likely be a reasonable representation that they have ascertained "the whole truth" from you.