I don't see why we should allow for the possibility that the ratio of cheaters would stay the same with or without the surveillance. That could only be true if the ratio of potential cheaters were so low that the threat of detection introduced by the spyware can't reduce the ratio further.

We may hate the software on ethical grounds, or because it degrades the exam-experience on many levels, or because its use can be considered abusive, but obviously it has an effect in the intended direction.